To hold the sidewalls to front and back, a rabbet and glue with some pin nails does the trick, though assembling it isn't a neat process. Front and back can take a dado to capture the bottom, but the sidewalls are only 1/8" thick, so to hold the bottom, I made tab-in-slot features with several 1" long slots in the sides, and with a dado blade cut into the bottoms (as a clamped-up stack) to form the tabs.
The problem: hardboard isn't what my tools are intended for. A 1/8" diameter carbide router bit, and a Rotozip-style side-cutting drill, make ragged slots (and it's not clear that the steel tool isn't overheating). A table saw leaves a furry edge (OK, that cleans up, by hand, with some sandpaper, but... it's a nuisance). I can apply a bit of shellac on the furry edges, then iron them flat with the old Proctor-Silex appliance, but one must be careful about the glue-surfaces.
Oddly, a brad-point drill makes a perfectly clean cut (to start the slot), so there's ONE tool that isn't a mismatch to this material.
My two questions: is there a way to form those tabs that doesn't turn so much material into sawdust (probably not, but I've gotta ask), and HOW can I make a clean slot in what is essentially just heavy paper? There's metal punch/die sets that could do it, but the supporting press tooling isn't gonna fit in the basement.
I can imagine a nibbler tool to form the tabs, but bandsaw (I've tried it) is slow compared to stacks fed to the dado blade in the table saw.