FAKE BOLT HEADS
Buy Nylon bolts (McMaster-Carr pg 2999) and cut the threads off. It is $10 for a package of 100 bolts on average.
Could you just hot-glue nuts of the appropriate size to the intended surface, then fill the threaded opening with joint compound or something similar.
You can buy hex plastic rod and slice it up too, sizes you indicated are available and cost effective. If you were using really large "bolts", say to join "steel"(wood) I beams, you could shape your own wooden hex rod on the table saw.
I took a piece of 1x1, ripped it to a hexagonal cross section, then cross cut pieces on the band saw. It took about a half hour and looked great on stage.