I used 1/4 Masonite and put a layer of dynamat on top just for the heck of it. Worked great, fit 5.25 and the tweet. A 6.5 may fit, I really dont remember how much room there was. I do know that depth was a concern (I used MB Quart, they are deep)

2000 SVT Contour #1077/2150 MSDS Headers/B&M Shifter/H&R's/

1995 Contour SE V6 #????/Tons KnuProject, awaiting mass mods