Ted, I have Ford F150, extended cab. I can stand up 5-6 cbg's side by side, then put a beach towel over them, like a flag. Then, a second layer, another towel, then a third layer, and a third towel. I can usually get @ 18 gits in there, and a couple of amps under the seat. The rest of my booth rig, tarps, canopy, chairs, display racks, table,etc. goes in the bed. I have a heavy duty tarp and tie-downs to cover the bed load if necessary, but thankfully my displays have been fairly local, and no real inclement weather. Your results may vary. Good luck on your festival ventures.
Ted, one more thing about my experiences. I found that 15-18 gits fills my booth well, leaving room to move about and play. I know some take more, sometimes a lot more. I am talking about a 1 day event. Also, I make more sales after the event than during the event. Have a lot of business cards, and I also have a one page history of cigar box instruments, which also describes my building process (briefly), and two ways to get a hold of me later. Everbody that stops to talk is offered one. Sometimes the inspiration to purchase, or to think up a custom gift comes after the event. Again, my personal experience ,others do vary.
We have cardboard boxes custom made, so they are good for storage, transporting and handy for the customer to take away a neatly boxed item. They are not that expensive (about $1 a piece). The only downside is that they are more bulky than packing in bubble wrap. We sometimes make simple sleeves from bubble wrap just put together with packing tape - not as durable, but again useful for handing to the customer.
I use old jeans, yes jeans. A 34 or larger size works best. I cut them up the middle to make two and have someone sew the ankle end or just tie it off with string. My high builds go in heavy padded guitar cases.
This sounds cool, could you post a picture of what they look like?