There is a possibility that you could get the brace you want made with a 3-D printer. You would need to find a makerspace or hackerspace that had such a printer, and work with the folks there to scan your wrist and print a useable brace. Google your town name and makerspace and see if anything pops up. Some orthopedic specialists are making 3-D printed casts, so check to see if anyone in your area is doing that. This sounds like the kind of project many makers would be interested it doing, so good luck finding a group near you!
In any case, rest is the only way for these injuries to heal.
Ask the event organizers. They may welcome such an item, and will be happy to have you make them. You might even offer to make a given number, which they could buy for a reasonable price, and mark them up to sell themselves. That way you don't get stuck with unsold items.
One way to make cash sales easier is to round up the price to a dollar amount and take the taxes of on the back end, ie. after you get home. That way you don't need any loose change. And you could price in $5,00 amounts and you would use fewer ones.
Unless you're Canadian, where you use coins for ones.