You might improve your traction if you shave your tires (to reduce its ability to carry high pressure air and loose salt, wrapping around to ground contact). You could mount your battery box as low as you can, directly in front of the rear tire. This will cut off airflow into that very high pressure air that the tire is dragging around the circle. Part of the problem is marbles from the front tire, perhaps?
Those big sidewalls are pretty good at throwing air under the front of a tire. Our roadster would drag salt, in the tread grooves, all the way around in front of the rear tires. The side of the body, in the upper forward quadrant of tire area, had the paint really abraded from that 180 mph salt meeting 180 mph air for combined speed of ....some big number! It was obviously the highest pressure point on the whole car.
This effect is so significant that many production cars are now using spats/spoilers in front of all four tires, and using tighter shoulder tread patterns. It starts to matter at as low 50-60 mph! I run a spat in front of my rear tire, and you can bet it will be deeper this year.
It had a LOT of salt stacked on it, last year, that blew back from the front wheel....and it was the hammered-in hard kind that you get at speed (and i'm only going 160 on my 650 pushrod bike).
Maybe a cheap and safe improvement for you. The way the rules read for open class, a rear tire spat isnt streamlining because its not ahead of the rider, and doesnt cover tire or wheel.
Regards, JimL