definitly try and do it urself, be alot cheaper
heres some pics of my custom door panels, mgiht give u some ideas to make ur side panel boxes
cut my rings for my 6.5s and little dowels to hold the ring in place( i used hotglue to place them)

stretched a old shirt over top and hot glued on the back of the door panel(stretch as tight as possible)

soaked it in resin(no fibreglass on the first coat)

sanded it

for some reason i cant find any pics of the fibreglass on them so here tehy are starting to get smoothed out

and i never finshed them as i got new black door panels, might try something different when i get the car back on the road
found some pics of my spare tirewell box aswell
lay tinfoil down and tape off all seems, i used duct tape but if ur going ontop of carpet it will leave some sticky residue left on it(this will be the back of the box)

put down a couple layers of fibreglass, once u get 2-3 layers thick you should be able to pop it out of the car and add more layers to it on a work bench or something(alot easier then doing it in the trunk)

this is where you would setup your MDF ring and dowels to mount ur sub, i wasnt going this route so i did i added a thin MDF boarder to it which was about 1.5 inches wide(hard to tell in the pics) and jsut simply mounted a MDF sheet ontop
afer you add the MDF rings/dowels you will follow the steps i posted above with the cloth etc