Thanks! It wasn't hard to make at all, just a bit time consuming. I spent a good 6-7 hours on the construction, then another half hour on sealing and carpeting the box.
-1 sheet of 3/4 inch MDF (2ft X 4ft) $11
-A pack of wood screws $2
-Some sort of sealer (I used dap quik seal and sealed the external seams after the box was put together- worked fine). $3
-Vinyl for the front $5 from JoAnn fabrics
-Carpet for the rest (had leftover from another project) Probably $10 max
-Speaker wire terminal $5 from ebay
-Cordless Power Screwdriver
-Dremmel with sanding wheel attachment (used this to make the indentations on the sides which will be necessary to fit the box in) You could also use a router.
I basically made the front part of the box first- the part that sits into the ski pass. From there, I knew the length and width of the ski pass opening into the trunk, so the only calculation left was the depth necessary on the rear part to give me my full internal optimal volume of .35 cu ft. I think my final depth of the rear portion was between 7-8". Once the two portions of the box were completed, I screwed the rear portion to the front.
I would also note that I chose an 8" over a 10" because, although a 10" will fit in the ski pass, the 10" would require a greater volume box.
Finally, to avoid rattles and the box moving around, I secured it in place with simple L-brackets into the existing ski-pass bolt holes, and reused the ski pass bolts. Worked perfectly and the sub box is bolted down nicely. No rattles, and great bass.
I would guess the box weighs about 15 lbs with the sub installed (total guess though).