Is there anyway to fill in the gap that is under the rear seat delete kit? Because i have two subwoofers on the kit but the open air under the kit makes the subs sound weaker (Open Air Effect). Does anybody sell something for that gap? or can just use a whole bunch of towels and smash them in filling in the gap, lol.

 

acoustic foam?

 

Given you have speakers there that require some form of enclosure volume, why not use the polyfil they put inside speaker boxes? You can pack it loose or dense in order to vary the compliance of the enclosure. BTW, are you sealing the kit, as any air leaks will impact the acoustic properties of the "enclosure" you've made with the floor pan and the rear seat delete panel?
I just filled the space with expanding foam (in cans at HD and other places). Makes for a nice acoustic dampener for road and exhaust noise.

 

If you have some room you could create a fiberglass form of the floor metal and then seal it to the actual delete kit. It really depends on your skill level.

 

Those answers sound good to me. Someone told me the other day that I should use concrete, lol!

 

+1 on the fibreglass. You could also make a box out of mdf and glue/bolt it to the RSD.