First...some advice. NEVER immerse your cat trailer in the water, NEVER. You are asking for trouble. Sure powerboats do, but how many times have you seen them broken down on the side of the road missing wheels, broken supports or non-functioning lights? My previous trailer was 18 years old, made of steel and had over 200,000 miles on it. It was well maintained, never put in the water and never had travel issues. My current trailer is an aluminum Trailex.

You can use a ramp and back the trailer to were the rear support is just above the water. Lift your bows and the whole thing slides right off. Repeat the process for retrieval. If your having problems getting on the trailer, mount a winch on the front mast support and winch it on. Get a set of beach wheels. You can launch from anywhere (ramps too) with beach wheels and roll your boat directly into and out of the water.

Regarding a sailbox... my winter project last year was to build a new water tight sailbox. I used a "shoe box" design with a removable top. The box is 12 feet long in order to accommodate a fully assembled Tiger spin pole. Since you can't get 12 foot long plywood, I used 8 foot pieces, 1/4" thick and offset laminated them together to make a 12 foot length. After the box was assembled, I glassed it with polyester resin/6 oz. glass cloth and finished with a marine grade paint. It's totally weatherproof and will last many, many years.

When I die, I want to be buried in my sailbox