Use 2x4s probably 2-3' in length. It should fit over the frame rails (at the rear of the tank). All you do is shift the tank to the right a couple inches, with the elevation of the 2x4 and that space, plenty of space to disconnect the fuel line.
Remove tank bolts. Lift the front of the tank. Insert the 2x4 standing up, not laying down. The 2x4 should now be covering the frame hole locations. Lower the front of the tank down, the mounts should not be resting on the 2x4. If you had long enough bolts, and drilled holes in the 2x4, you'd actually be able to bolt the tank down.
Next gently lift the rear, insert the 2x4 standing up under the ount. Again, lower the tank carefully.
It's stable enough that unless your 90 lb lab comes running up and jumps on your bike it's not going anywhere.
Carefully shift the tank over a bit, like I said, 1-2 inches, honestly you could probably do more. It'll be fine. Disconnect the line.
Setup where ever your going to put the tank. In my photo the belly of the tank is resting on the 2x4, keeping the outlet line off the floor. Pull the tank off, set it on whatever you got setup. What I got setup, my tanks not going anywhere unless someone knocks it over.