Sir Pioneer 308 is not a very good SUB for SQ, however it will sound good in a sealed enclosure.
You should be better with a Kenwood 3012, Kenwood eXcelon 1024 10", JBL GT5-12, AlpineType E or JL 12WX, whichever you can find.
You can make the dimensions as you want, just take care of the strength and wood/MDF used, it should be of best possible quality.
If you cannot get your hands on MDF/Lasani, get any other good plywood, at least .5" or as thick as manageable/affordable.
Seal all the corners with a carpenter glue (yellow color).
The smallest size is generally good for SQ.