There is nothing more frustrating that having to futz with bumpers at a competition.
Thoughts.
1. Baltic birch plywood. Not cheap 5 ply wood from the home center.
2. Corners cut at 45 degree angles transmit impact loads better than 90 degrees, all things equal.
3. Use sheet metal corner brackets on the outside of corners. Simpson A23 or equivalent work well.
4. Use a large diameter bit (e.g., 1") to cut recess holes for hardware that extends beyond the plane of the chassis, so that you don't have to have perfect alignment of the hardware/holes. Don't put so many holes that it impairs the integrity of the plywood -- that's what the inspector will be looking for.
5. Use a pneumatic stapler.
6. Pay attention to minimum clearance between ground and bottom of bumper. In 2014 this was 2". You will want to allow a bit of sag in the fabric so don't put the backing plywood right at 2".
7. Many fabric stores carry a polyester gabardine that is suitable for bumper use. Make sure that the fabric you select doesn't have any stretch to it.
8. Pay attention to color. There are often swatches of red and blue fabric in the kit of parts. It's not an aesthetic decision -- you want your bumpers to look like everyone else's. Don't send Grandma* to the fabric store to "get some red fabric."
RockyWoods has the 1000D Cordura in a coated version for $12 a yard. "Red" and "Royal Blue" are the colors.
* or Grandpa