you have hit all the roadblocks that stopped me from making the same thing last year. how to do the wheels? getting the ski on and off is easy its removing the wheels mid tilt. or having clearance to mount with wheels.
I have an idea but the thing is the more you add to it the more expensive it becomes until you get to the point where its no longer cheap and simple.
as it is now just the way you have it, there is plenty of time to cut and weld it up and by the time you make bunks and carpet them, drill holes for rec. pin add hardware its not cheap anymore or quick and easy unless you work for 5/hr.
easiest way is to bring a cart.