A) I went back and read your original post. I kind of like the hinged plexiglas idea. Thin plexiglass is pretty easy to work. If you want to bend it into a box or just an L you can make a heater out of a piece of electrical heating element (like from an old toaster). Stretch your recycled heating element out straight on something non-flamable and non-conductive like backer board or concrete siding. If you are electronically inclined, throw a dimmer switch into the circuit to control the heat. Carefully heat the line where you want to bend your plexiglass so that just a very narrow strip is softened, then make your bend. You could use small squares of plexiglass to form shell holders that hold the cartridges by the rim/extraction groove. Glue the shell holder in, not your cartridge.
B) On the topic of embedding in resin, I really dont approve of doing this to collectable cartridges, but if you just want to make a display of your shooting ammo, then knock yourself out. Somewhere, I have seen instructions for embedding objects in resin and I am pretty sure that it is done in two pours. First you build a form to hold the resin on a sheet of glass. The idea is to form your viewing side on the glass. Pour about a quarter inch (or whatever depth you want) of resin into the mold and let it harden up enough to support your cartridges. Place your cartridges face down on the first layer, then fill your form with a second batch of resin. Let it harden up, remove your form and pry your work of art of the glass (they make release agents to prevent undue sticking). Then trim and polish to your hearts content. The biggest problem I see is dealing with air bubbles, and you are on your own there.
C) What do you collect Josh? Do you think you can be satisfied with less than sixty cartridges?