I'm pretty new to these kind of builds, but I do a lot of exterior/interior/engine work to 4x4s as a hobby and I'm sure the skills somehow overlap. I find a lot of DIY stuff often does. I've fixed cracks in plastic items using plastic welding a couple times, but I'm no plastic expert.

Anyway, I have this polymer handguard. I already need to cut it back quite a bit because I already have this one, it's brand new, and I'm poor, haha. So what I'm trying to figure out is this. The hand-guard (like nearly all others) has a type of "skeletonization pattern" for like M-lok or whatever. If I wanted to fill some of that in and sand it and really make it exactly how I want it what should I use? I'm asking here because there might be some kind of epoxy that is commonly used by you folks for this purpose. I want to find something that could (theoretically) allow me to make all the walls of the hand-guard solid if I wanted to. I probably won't do that, but I want to know what I should use if I wanted to. Is there a way to do it where it could then be drilled out again and retain it's structure as well as the unmodified portion? If so, then I could give it an M-lok slot further back how I need it. I mean, maybe I should treat it *exactly* like plastic-welding, but I'm not 100% sure of that so please let me know. I don't mind taking my time and investing a lot of time into it. Maybe I'm just a masochist, but I find that kind of thing fun.

Thank you

These are probably cheaper and easier:

https://guntecusa.com/product/gen-2-m-lok-rubber-neoprene-insert-covers-with-protruding-grooves/

I would try one of the JB weld products. You would likely need to paint or coat it afterwards to get a uniform color.

An epoxy of any kind will likely be ok after the 1st cure period, but it will soon fail.
Here's why. Epoxies are exothermic, and when they cool down they will shrink a tad, some more than others.
If you are filling a hole in a relatively thin material, it's likely to just fall out.
I guess you could maybe use a screen backer where you take fine hatch metal screen (screen door) and then cut small patched to cover the holes from inside, use a hot tip of screwdriver to slight push a few edges of the metal screen into the plastic just enough to hold it, then install the epoxy.

There are several types of epoxy to use, liquid and paste. Perhaps paste is a better choice? Sounds messy and interesting.

Since I have never tried doing such I really don't know what the actual results will be, but a low temp hot glue might do better than an epoxy for the scenario given.