There is a photo attached of one of the smaller boxes I had. This one was also fiberglass (100% in this case too), it was covered in vinyl.

The box shapre resulted in around 0.7 cubic feet if I recall correctly (but that is before taking into account the speaker displacement). It is similar in design to the one you linked, except that it doesn't have the extension out of the corner of the hatch (was trying to avoid the box taking up 'usable' space in the hatch).

In this photo, I had a Fosgate 10".. initially I wasn't too happy with the low end response. I wound up tweaking the speaker by mass loading the cone (glueing weights to the back side of the cone). This flattened out the response and the speaker/box system has a lower cutoff point... but of course the efficeincy went down by a similiar amount (the other trade off is shortened speaker suspension life and possible cone misalignment if too much weight is added and the speaker is mounted vetically... although I had no problems).

I later installed the illusion audio 10" in that same box and it was a little better. But with either of these, I never had enough output in the 20-30 Hz region (but it did still hit about 126 dB at about 40 Hz in IASCA competitions.... almost the same as my 12 hits at 30 Hz).

If you build yours like the one in the linked photo, you should have significantly more enclosure volume... more than enough for a reasonable 10 to be happy with.

The two 8's I had were in low profile box about 9" deep that followed the contour of the rear of the hatch (looked like 6" tall shelf sitting on the floor... it was wood, but still light enough to be reasonable to remove to access the spare).