I recently had a to design a box 10" Sub box to fit in the back of a convertable, a very small area...
For you, you can go with either 2 boxes or one big one, whatever space allows.
First, sketch out your design, take into consideration the volume of your Subs and add that to your volume to form your 3 cubic feet box. This volume should be in your instructions or specs. You didn't list that spec.
total volume of desiged box = 3 cubic feet +(volume of Sub)*2
3 cubic feet + (.25 cubic feet Sub)*2 = 3.5 cubic feet= 6048 cubic inches
so lets say you want volume of 30"*15"* ?" = 6048 cubic inches
30"*15"*13.44"=6048 cubic inches
so if you used 3/4 wood you would want to over lap the ends by 3/4 of an inch so take that into consideration. It the top and bottom over lap the sides and two sides over lap the ends...
top 30.75"*15.75" w/ 2 12" holes cut out
bottom 30.75" *15".75"
long sides 30.75"*13.44"
end sides 15"*13.44"
Now that you have your design, go to your local harware store and find the thickest and heavest wood/material you can find.
I highly recomend using a table saw to cut it, you want perfectly straight and right angle cuts.
once you have all your pieces cut out, put the box together with wood glue, then you a nail gun to make sure it seals nicely, (or use finishing nails)
When the box is dry, take some silcone and cauck all the corners inside to get a nice seal. drill 2 holes for your wires and then put them in leaving engough to work with and glue them in with siclone. We are are going for an air tight box. Let dry
Paint or carpet the way you like it.
Fill the box with fiberglass insulation like you would find in your attic, place in your speakers with there seals or glue them in with silcone. Secure them with screws.
This method should give you a really hard punching bass box. You will really be able to hear the bass kick drum and all it's glory along with a nice clean bass sound. Volume may be reduced some, compared to a ported box, but the ported box will not have the hit of the sealed box.