Might I suggest that you use fiber board? It’s fairly cheap (about $5 for a 4×4 piece at the hardware), sturdier than cardboard, and you can put pins in it or hooks. It’s also not terribly heavy, but can take some abuse.
At the craft/art shows I frequent, many of the artists cover cardboard, foamboard, plywood or use jewelry display cases (they have lots of velvet-lined boxes within the case). I’m sure there are lots of other suggestions out here.
I live near a local cabinet maker who sells drawers for a couple of dollars. I get the shallow ones, put in dividers and put black velvet “pillows” on the bottom of each space to set off the work. Right now, I’m just protecting my work; when I get some courage (and a few more pendants), I’ll be set for a show.