My Ford supplied bed extender was nearly 3/4 inch too wide (3/8" on each side) to fit between the Access side rails. It seemed I could use the toneau cover OR the bed extender, but not both. A fairly simple fix gave me the use of both at the same time. Here's what you do (Please double-check your own measurements for your particular truck, extender and Access cover):
The rails and nylon block are easy enough to trim. I used a cheap electric sabre saw with metal cutting blade for the rails, followed up by a metal file to smooth the rough edges, and a band saw to neatly trim the removable black nylon latch blocks. That part of the operation took only 1/2 hour or so..... (Note: Access uses 2 different locking systems. One system uses 2 sliding "bolts" to secure the left and right corners. The other system, which my cover has, uses a center mounted spring latch **** that simultaneously releases both tabs at the same time.. With either version you have to extend the tabs/bolts outwards a bit, so that they will still get a good "bite" underneath the newly modified side rails). My version is fairly simple to modify: cut the latch wire in mid span (it will need to be lengthened later). Drill new small pilot holes in the rear crossbar, and relocate each stop screw so the latch assembly is 3/8" closer to the modified rails, where they will once again firmly snap into the locked position when the Access rear crossbar is closed. Lastly, use a piece of braided wire and wire crimpers (from fishing tackle shop) or 1/8" copper tubing from a hobby shop to lengthen the recently cut latch wire by approx. 3/4 inches. The longer wire will make the lock operable again. The pictures will hopefully make all this clear. The total project time is maybe 75-90 minutes, and can be done by anyone with moderate skills, and a few tools. The bed extender now swings out just as before, even with the Access rails in place.
Larry in SF, CA
Link to pictures: http://www.pbase.com/gslarry/access_modification