+1 just find a good car audio palce nearby , must be one somewhere close ish and get them to make it and buy the sub from them if you feel you cant make the box yourself.
the under strut is a very simple design , bacialy a box made to the right size for your sub with panels at the front and back to match the struts curve stuck on.
re volume , if your not going ported then as long as its not say 10 litres either side of the spec it wont make much difference.
fiberglass is the prefered material for audio since it cna be made to any shape and strength by just makeing it thicker, also alot lighter. getting an FG box made will be considerably more expensive but makeing one yourself works out about the same since its a cheap material but very labour intenisve vs just cutting some mdf and putting it togther.