Found a few on Ebay, mostly for Rangers. Couldn't locate an "after market" builder/supplier. Have you priced one from the manufacturer ? Probably pretty high.
Ran across this snippet on DIY,
I built one for my R82 and one for another guys 519. I used 3/4 inch marine grade plywood doubled, glued, and screwed together then cut to fit between your rod lockers. If you want an exact flush mount, add carpet to the top and sides. Please note if you are going to carpet, you will need to cut the plywood a little short in order to accomodate the extra space needed for the carpet. Usa a piece of scrape carpet on the front and sides as a guide. Be sure to use 1 1/4 inch wood screws and spaces them about 3 inches apart all around the perimeter and down the middle once you have the plywood cut to the right size. This ensures that the plywood will not seperate due to the elements. If you carpet, glue the carpet in place with 3M exterior carpet glue on the top and sides. Use staples on the sides to hold the carpet in place until the glue dries. Total cost about $75 plus your time. Takes about 1 days time spread across 2 days (need time for glue to dry).