I'm not sure I understand why the trailer box has to get wet, unless there's heavy wave/wake action at the ramp? The box is ABOVE the trailer frame, and the rollers and cradles are only 2-3" above that. Is there any beach cat that will be damaged by a 3" drop to the water?

A 4,000 lb. motorboat HAS to be floated off the trailer, but a 300-400 lb. beach cat? As you roll it off BY HAND, the cantilever provided by the portion of the cat still on the trailer means you (and your back) will only experience maybe 30-40 lbs. of load... you need more muscle than that to tug on the mainsheet.

I had a Dilly Cat-hauler w/ a 4' x 8' plywood box on it. IIRC, the rear rollers were on top of the trailer's rearmost crossbar, the taillights were on the backside of that rearmost crossbar, and the rear of the wooden box was also even with the backside of that rearmost x-bar, pretty standard setup... (it had rollers up front too... lack of cradles didn't seem to bother the P-16's tough keels one bit.)

(FRESH WATER LAKE): I'd back down the concrete ramp until that rear crossbar and lights were just about to get wet. Do not remember it being a very steep ramp. The Prindle's keels curve up and so were maybe 8-10" off the water. Unstrap boat, grab BOAT'S rear x-bar and roll back until balance/tilt point reached. Since nearly balanced, only a few pounds of weight supported by my arms. Gently lower stern into water, step around to the side, and continue sliding off trailer. Never got wet very far above the knees. Keels in no danger of scraping bottom... (rudders UP, nach)... box and lights: DRY.

The only part of the trailer that gets wet are wheels (and leaf springs), but that's why Bearing Buddies were invented, i.e. the spring-loaded piston keeps the grease packed tight, leaving NO ROOM FOR WATER.