I just finished building a sub box for my 96 Taco as well. It a bandpass, but you could use the same box dimensions and put the subs on top or the longest side behind the seats for that chest compression effect.
I'll attach it. I used to just use 2 seperate boxes but it filled the whole backseat area. The size of the box is just large enough for the 2 12" pioneer subs I have, but you could use 1 high quality 12" or 2 10" 's. My Truck pounds, but I don't play it to impress, so my bass levels are normal. Its amazing how many ppl buy subs so complete strangers can notice them in their Sunfire, even if it completly drownds out their factory mid-ranges.
If I built this 10 years ago I would have made it 52" wide, but I got sick of squezing a passanger in the middle, unless she has nice legs. This box leaves enough room for a rear passenger to ride without being crippled. Its a few inches narrower then the driver seat and you can turn while in the back which means you don't cramp up in 30 mins.
My speakers used to be facing up, but my pooch loves to ride on top of the box which got them dirty and I couldnt turn it up when she was up there, so I went with a bandpass box and added a fold down extention so she can lay in the middle and watch the road behind the seats. She's my wingman, I even got a seat in the boat for her. lol
If you only want a little bass and value every inch of your rear seats, you could put some 10" subs in the under-seat compartment. It wont rattle your mirrors, but some ppl like the hidden style. If you do put them in there, spray some expanding foam sealant in the edges of the compartment to get it air tight and stop it from rattling.
The compartment sizes and ratios are dependent on your subs, as well as port length/diameter. If your really serious you can calculate the best size for box compartments
I use 3/4 or 5/8 MDF wood, and make it strong as hell with lots of bracing and inner supports. Use lots of glue and screws. You could use the rear seat belt to help secure the box, and make life difficult for stereo thieves. I'm not sure why you would make a box out of fiberglass since it would have to be so thick to be as effective as MDF. If you want the best get HDF (High density fiber). Its a waste though IMO. I'd only be concerned that the fiberglass would reverberate which actually dampens the bass. Thats why 3/4" - 5/8" MDF is used by pros. But suit yourself. I'm a lot more comfortable with the saws then the resins myself.
It doesnt matter if you build the box, the sub will sound like GARBAGE unless it is speced to the specific sub it is using. Why? Each sub is different. It requires different airspace. The sub doesn't make the sound, the box does.
That being said, if it worked good for sub A, it will not be the same for sub B.
Spec out the box. I use a program called WinISD when I design out boxes.