1) I car-top my 'Fish, so it rides smoothly—like the vehicle it's on.
Most Sunfish trailers appear to be "over-sprung", which means that the too-strong leaf spring imparts severe road shocks to the hull—both on "hit" and rebound. (Sometimes several "rebounds" with each bump.) Repeated road shocks might be contributing to the loosening of interior foam [flotation] support blocks.
Attaching an automobile shock absorber parallel—diagonally—to the axle might be a good innovation in a homemade Sunfish trailer plan. (Even a used shock absorber would be an improvement. IMHO.)
2) I'd carry the board and rudder in the trunk of your car—away from the trailer. Though Sunfish and their trailers are seldom the target of thieves, there's no sense in giving them a $300+ incentive to steal yours!
3) I welded a chain link to the inside of one of my former trailer's wheel. When left unattended, I'd padlock a chain to the frame of the trailer—unseen by most passers-by. If thieves drove off with the trailer, you could follow the streaks on the road to where they eventually abandoned their "take"!