I actually have a trolley for the garden etc made from a sheet of plywood plus a set of 5" castors, just bolted on, 2 are fixed the other two steerable, the pull bar is a length of timber and is connected using another castor with the wheel replaced with the draw bar, a 8" bolt through the handle makes a pull bar. the castors were obtained from an abandoned damaged diy shop heavy duty trolley that I spotted lying around., with this the base can be made up with a cradle to take your model, and built to a size to take the boat. maybe then all you need is a ramp to wheel it into the car.