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Not at all! I've found that most professional install places prefer to just screw it straight the back of the sub box. I've done that for years, until I got my KJ and found that there was plenty of room under back driver's side seat. It's actually easier to run wires there too, because you don't have to run them all the way back to the cargo area (other than your speaker wire). And, seatbelt bolts make a great ground location for the amp!
The only problem you could really hit is if you have a very small sub box (which you do) and a somewhat heavy amp. But it seems that you've figured that out by strapping it in place! Tip: Velcro on the bottom of the box and floor also helps keep it in place and looks a little nicer.