I was thinking for something stronger, I'd might lay down a pair of 5' vertical E-Track strips front and back. It seems to be those might be easier to securely install in the bed of a trailer though I'm not exactly sure how... And it would give me the flexibility for different length cars, adjusting the load balance and of course.... JUNIOR KART placement.

Also thinking of using vertically mounted vertical E-Track for the sides to form the basis for an adjustable height tire rack. Again, the E-Track seems to me to be the ticket for distributing a load across a relatively flimsy working surface. But I'm never used them.