Ok, hear me out on this one. Why dont just add and attachments box?
It would be something like this :
cons - Less drop overall, No weapons you can scrap/use
pros - You only get attachments, and have a bit higher chance to get better ones.
OR
It can be made as a parody for main custom box, where you can choose weapon you want to get attachment on, and set a lvl limit for dropping attachments, For the sake of the balancing, it can be limited by setting minimum attachment lvl to 80 or so, and raking up the price for box to the sky like it done on main