Fibreglass wheel well build isn't that difficult. Time consuming yes but not difficult. Plus if the box is built too small you can just stuff the box with polyester filling which will trick it into thinking it is in a bigger box. Using most of the well though I reckon it would be about right for a sealed box.
Also fairly easy to calculate the volume. Basic highschool maths, as long as you can remember your volume measurements involving Pi you can work it out.
Just give it a go, done a few builds now myself involving fibreglass. Not hard to do just time consuming. For a boot build like that though you could just get a polyester material and lay it in the well and do that as your first layer, then some chopped strand over it. Reckon you can have the mould done in a day. And the top done the next day. One bit of advice, when you lay the masking tape down etc, put some tin foil between the tape and the matt. Will make it easier to get the mould out. Also worth giving the wood a layer of resin over it, as wheel wells can get a build up a lot of condensation etc. So putting the resin on the wood will basically make it water tight so it won't rot.
Seen people make MDF boxes that fit in the well before and they wonder why they start sounding s**t after a few months. The carpet gets soaked and MDF just ends up falling to bits with the moisture.
