You could mold one with the right kind of molding compound. Take the mold material for my pistol grips (yes that is still an active project). Pour the goop around the part and you have a mold good for some time. It is pliable enough that it pulls free of the form and if you baby it you can avoid ripping it apart. A multi-part mold could be done, but you have to plan that better because you must have some sort of index point so you can rest the pieces the same way every time.
Problems I have encountered with molding are -
(a) the fact that bubbles will ruin your work. QA is the name of the game. Vacuum de-gassing chambers are spendy. Re-doing a mold is pricey and time consuming (takes about 1 day for the mold to set). I have several trash molds that are tweaked due to bubbles, etc. and that molding plastic is pricey. I have a shoebox full of "seconds" that honestly I don't want to give to anyone.
There is a newer kind of molding compound that apparently doesn't have the bubble problem, but I have yet to look into that. If it delivers as promised, it sounds like the best path.
(b) a template from which to mold. What do you do - build the prototype from wood or foam then mold it? You'll have to use bondo or some other filler to take care of all the voids so you don't get "extra" pieces on the final piece.
Give me one of those real Bizon mags from Rusmilitary that are demilled, and I can use bondo to build up the feed lips, and presto there's the template for the mold.
(c) the molding material. Most molding material is clear or white. To color it you must add a pigmentation of some sort. Then you don't get a true black, more like a dark dark grey. Honestly I think a fiberglass material might do better.
The black heat-resistant material I am currently using has a pot life of about 2 minutes TOTAL. After about 1 minute, it starts to set up like cold molasses, then that's it. Makes for some really interesting "poured from a cup" desk art when just doing pistol grips, but for a large part I don't see that working. What we need is a good plastic that doesn't require pigmentation AND has a longer pot life. 5 minutes would be better.
So is it doable, certainly. Just takes a lot of work, and the setup cost is pretty high.