Your TJ gas tank is secured via top straps to the bottom skid and the skid to the frame. You could redo the top straps to secure the top of the tank higher near the floor, use other straps on the bottom to secured the tank, and then add a separate skid plate altogether.
This is the plan I have. To secure the plate, I'm looking at inserting into the frame a 1/4x1 plate with nuts welded to it. Then with proper location, drill through holes to match the plate. The skid plate would reach to the frame and be bolted to the inner plate. I did this on my Bronco but it was much easier as the frame is 'C' not enclosed and didn't curve up right away.
Only problem I foresee, the holes near the rearmost crossmember are reserved for strengthening aftermarket bumper attachments and tow bars. So I've drilled an access hole on the rear most crossmember for the frame for the nut plate.