Get one out of an XJ at the junk yard, you'll only need the bottom pan/cushion. Only four bolts hold it onto the frame. The metal pan on my driver's side was cracked parallel to the frame on the outboard side causing the sag, got one for less than $10 at the local pick-n-pull.
I put some plywood between the foam and springs, not the most comfortable on long trips but it was temporary until I got a set of buckets.