[dsraven]

if you have a console an under seat heater will work as well. place the main unit under the seat and run the tubes for the defrost through the console.
if I remember you don't have a console or plans for one
a heater from a donor heavy equipment unit, backhoe, etc, may work too. if you have a surplus store you may find something a well.
b uilding one would be my go to as the stock unit is marginal if you found one. I would find a heater core, find a fan unit, build a box around the core, mount the fan and install some outlets. done deal. thgin gauge aluminum is a great medium to work with or sheet steel if you wanna keep the costs down. you can bend it over the edge of a plank to form a square tube around the core, then build some end caps that will rivet onto that with holes for the core inlet/outlet, place some "shelves" on the inside for the core to sit on with room for some canopy tape to insulate against mechanical damage, then mount the fan and cut a hole for air flow on one side and then opposite that cut the holes for the tubes to go.
BOOM
done in a day