Mounting amps upside down is one of those debated topics that usually has to do with how they disperse heat...but with class D amps being as efficient as they are it is (in real world) not really a concern but I can appreciate your wanting to stick with JL's recommendation. I believe alpine says the same thing. I've had that MRX-F30 mounted like that for years with no issues...and just recently moved the PDX up there to make room for my new sub enclosure, and it is handy to have easy access to the gain's, crossovers etc
I like what the guy did in that link, but I do occasionally take advantage of the rear seat pass through. Other than maybe somehow mounting them in the spare tire area...that amp board idea looks good.