3/4" MDF will work best here. 3/4" cabinet grade plywood also works (13ply).
The MDF is the most cost effective though around $25.00 per sheet. If you use screws you must pre-drill your holes. 1-5/8" coarse thread drywall screws work well.
With the dimensions that you have given for your space here is what I come up with for an enclosure size.
EXTERNAL DIMENSIONS:
Width: 35"
Height: 20"
Depth 24"
Woofers would be on the 20" X 35" plane as would the vents.
The enclosure would be sectioned in half internally.
Each INTERNAL HALF of the enclosure would have dimensions of:
Width: 16.375"
Height: 18.5"
Depth: 22.5"
The vent dimensions in each half of the enclosure will be:
Height: 18.5"
Width inside of vent: 1.625"
Depth: 39"
The vent will need to be labyrinth style to completely fit it in the enclosure along with rounding off the edges of the vent pieces internally to prevent sharp edges.
After some woofer, bracing, and vent displacement, net internal volume ends up at 2.704ft^3 and a tuning frequency of 25.97hz. Small compromises but one you would probably not hear/notice. Because of the size restrictions you will lose a little bit of low end response but it will allow it to get a bit louder.
Please look at my build threads in my signature with vented enclosures to get an idea of how to construct the enclosure properly with adequate bracing. These photos would also help out whomever is helping you to construct the enclosure.
I would offer to build it for you for a fee but shipping would be outrageous on 90+ lbs of MDF.