I've never built a trailer, but Cliff Jacobsen covers building a canoe trailer in his book "Expedition Canoeing". His design sounds very close to what you describe.

My Dad has a four-place canoe/kayak trailer by a company named Magenta. They ship the trailer UPS in three or four boxes, and you assemble it yourself. Assembly was easy. You could probably also add custom a gear box for minimal additional work. You might price out that option, and compare with the time and cost of a build-your-own job.

Ranger