Seal off door for 8" sub or build an enclosure within the door?

Ive found a good deal on 2 8" subs and i want to put them in my front doors, they call for 0.5 cubic ft, i figure i could maybe build a fiberglass enclosure within the door or save myself the trouble and use 1/2" gasgetting foam and other types of sealents to seal off the door. I would also do 2 or 3 layers of sound deadening on bothe the door panel and the door. Think i could get away with this or should i build a fiberglass enclosure? The subs are rated for 200 watts rms each. I will also be putting polyfill on the inside of the door panel and door. If you dont like the sounds of this do u have any sugestions for some loud 8" ib subwoofers for under $150 for a pair?