I like boat trailers that have other functions too.
One way is to adapt another type of trailer - like a conventional box trailer at least for smaller boats.
An alternative is to go for something very "bare bones".
One thing that makes a boat trailer more useful for different boats is to have a longer drawbar than normal on the trailer.
So if you are having a trailer made - adding a foot or two to the drawbar may make it useful for bigger boats too.
Most of the boats I design are pretty light anyhow.
With the dual puddle duck setup note how we have used a frame that fits in the top of the box trailer for the lower boat - no chance of that moving at all - and then use two padded cross pieces with holes in the end for the ratchet tie downs to pass through. That can't move much either.
The frame trailer picture right is for the flat bottomed Goat Island Skiff that has two longitudinal runners on the bottom of the boat.
The cross pieces line up with bulkheads inside the boat.
If you are using a cross bar across the top of the top boat I would put holes in the ends of that for the tie down too.
The whole lot sat down very happily all the way to the recent Stansbury/Port Vincent Wooden Boat Festival.
PDRacer Trailers including a single trailer too 
One pic and a little comment about the frame trailer
I would normally put plywood over the trailer frame before adding the bearers that support the boat to reduce the risk of stones chipping the hull.
This is Robin Badenoch's trailer - it is one of those buy online and we'll deliver at the end of the week and we will assemble type packages.
PDRacer & Goat Island Skiff Trailers 
