Don't remember where but saw some one had mounted something (wasn't a pelican case) using some old cut up rails and a set of hifaxs. Guessing that would require a decent amount of work.
Another idea would be trying to use the linq system. I have never seen the mounting parts up close but I am guessing it could be done.
Alsoooo... may be more work than the average guy wants to do but you could dovetail a couple tracks through a piece of HDPE, mount that to your pelican case and then use mountain addiction rails.
Orrrrr you could just not worry about it, save some money and bolt that thing right up. If your afraid of it getting taken throw a padlock on it, in order to unbolt it it would need be opened up.
I love the pelican case on my Axys, cost half and works twice as well as any bag I have ever used. Still got its quirks- the latches will occasionally get snowed/iced up and require a little bit of "work" and the fact that lid is 2" deep rather than flush with the top can make getting it shut when its full of chit a PITA. But its bombproof/waterproof storage, and works WELL. Good luck, post up some pics when you get something thrown together this fall.
haha but it's the best system by far.
