That box sounds good, and 3/4 of a foot air space is fine. Sealed enclosures work better for smaller boxes I know from experience becuase I have a small enclousre too. When I built it, I left it open at first just to see how it worked, but I thought it sounded like crap. Once, I sealed it, it was so much tighter and cleaner. I think the bedliner material would act kinda like dynamat which might be beneficial to aid in tigher bass. Hope this helps you.
Just because I don't say it, don't mean I aint thinkin it.