Has anyone here tried to make a custom or cloned rabbit hole? What kinds of mechanics are involved in how the rabbit hole works, and the location of the entrance?

Also, are parts of the rabbit holes often invisible? I've noticed when placing rabbit holes in custom lots that even if it looks like there should be enough empty space for the rabbit hole, it gets rather finicky on certain sides of the lot. It might require a certain number of free tiles on the left, right, or back. So are the rabbit hole objects often bigger than they appear?

I'm asking this because I would like to design custom rabbit hole objects, but I'm wondering if it would be more practical to just design custom building shells and put rabbit hole rugs inside of them. Part of the reason is I'm working on a custom sci-fi themed world and I want to make custom rabbit holes that anyone - even those who don't have Lunar Lakes - can use and still keep with the sci-fi motif. I also have decrapified all my store stuff a couple years back and know that can present some problems with non-decrapped store content. I don't know if decrapping alters the store world rabbit holes as well.

If anyone has some tutorials on cloning and creating custom meshes for rabbit holes or building shells (I want to make some really cool sci-fi style buildings for my space colony) please post them here so I and others can learn.

It is possible, but takes a lot of time and effort. A lot depends on the meshes and textures you want to use. If they're from a different game, you may have to convert them into the appropriate formats .DDS and .wso (if using TSRW). You also have to move the whole building in Milkshape or any other program like that so that it fits the original entrance areas. You do it to make sure sims won't get stuck trying to enter the building. The number of groups also has to match. Sometimes you can delete the unwanted groups in TSRW if the original rabbithole mesh has too many of them. It's really difficult to describe it in one post. There are so many thing to take into account.

Alternatively, you may build you own buildings (not rabbit holes) and put the rugs inside.

I figured you could use a mesh with similar dimensions but with a different design and not deal with other factors. I would really love to see some step-by-step tutorials on custom rabbit holes and the like because I would love to make ones with custom names. Basically, ones after my great-grandfather who was one of the famed UFO contactees. I'm working on my first serious world and it happens to be futuristic, and well, I've decrapp'd my store content (bought and paid for with loads and loads of ads that I spam-window'd). Don't like the Origin or whatever other launcher program taking up RAM and CPU. And I know decrapp'd stuff can cause problems for those who don't have decrapp'd items.

So yeah, I want to make my own rabbit holes that aren't Lunar Lakes rabbit holes.

I'm still not great at modeling, but I want to know about the mechanics needed for rabbit holes so I can possibly make some with simple futuristic buildings. I want to make a good planetary colony with rabbit holes that would match the setting.

I think if you familiarise yourself with object modding in general, the mechanics of rabbitholes wouldn't be that far beyond understanding. EA didn't make them work with magic, after all.
It's an object like anything else, with bones that define what parts move during an animation and slots that define where sims go to perform those animations.

Taking apart an existing EA object is always a great place to start. Once you know your way around S3PE, there are few things that you can't learn to replicate
TSRW also has decent slot management, for actually seeing what is going on and where in the 3D preview.

I see, but I want to a tutorial on dissection of these objects. Is there one for large interactable objects like rabbit holes?

A lot of this stuff is trial-and-error, just experimenting until you learn what works and what doesn't- there isn't any one tutorial for where to begin doing that, but any object tutorials will help you understand what you're looking at while you do so.

I think beds and ovens are good objects to look at when learning how all these resources work together. They're quite basic, but involve many of the same kind of parts- having pieces that sims interact with and animate.
If you open a stove in TSRW and look in the Slots tab, you can see what it has going on in order to function the way it does. Three bone assignments, two routing markers, two container slots, three effects (on the one I happen to be looking at). If you click on these, they will show as little XYZ markers in the preview.
If you made a new stove mesh you would have to think about making sure it had these things in the right places so that it could function too.

The same concept would go on to apply to a rabbithole and the resources it happens to have.
If you clone an EA rabbithole with S3OC you can open it both in S3PE and TSRW, which each have their own insights. You'd also have to export the mesh (with either program) to view the bone assignments in meshing software.
I'd recommend working with the smallest rabbithole you can think of, as the larger buildings have very complex meshes. Those Lunar Lakes portals would be fine to use as reference, the script itself is all base-game, but the mausoleum is also quite a simple one.

If the documentation doesn't exist, sometimes you gotta be the person that looks into it. Worst thing that can happen is that you make something that doesn't work- but that's all part of the process, even for those who do know what they're doing

I'll have to try that out.