>>2079721PVC pipe, connectors and PVC glue to make frame that sits over transom in inverted "U" shape, then screw plywood to pipes that hang over the end. That should be OK for very small motor, but don't go anywhere you can't swim to shore safely. Bigger pipe less stress on your shit-boat, so about 1.5" schedule 40.
Total cost about $30 and total time to build about 1 hour using hand saw.