When I had my fiberglass cap on my old pickup, I used a stand that I built.
I parked the truck where I want the cap, I then sunk 1 4x4 at each corner (in a few inches front and rear) of the cap, then ran a 2x4 along each side of the vehicle. Then I cut 2 more 2x4's to go across the other 2. I would get inside the bed, lift up a couple inches on the cap with my back and slide the 2x4 cross ways along the front and back.
Drive out from under it.
To put back on, reverse process. Seemed to work quite well.