2001 chevy z71 sub enclosure

I have a 2001 chevy z71 extended cab (4 door but back 2 are suicide doors) and have 2 -10" memphis audio subs.  I have talked to numberous "stereo shops" and all are but too happy to build me a custom box.  I purchased one of the pre-fab down-firing boxes and i guess it did o.k. but i recently heard pretty much the same set-up as mine and his stereo made mine sound like a playtoy.  The only difference in our stereos were the enclosures.  I have searched on the web, but nobody has info. for the novice like myself who wants to save a couple of hundred bucks and at least try to build the box myself.  Does anyone have a good box design that will fit under my seat.  The box i mentioned was a front facing enclosure with slotted ports.  I would love to give it a try, but I just need a drawing, diagram, dimensions...ect....anything that shows me the shape, correct port dims, airspace...you know......

not sure if i makes a difference, but the subs are only reference series (15 PR104D) and I will be pushing them with a Phoenix Gold X50 500 watt amp.  Any info will be greatly appriciated.

Hookemtim