Are you sure you were using actual MDF? I've never had it fall apart on me :/ Also bro, lol you should make the $40 investment in a jasper circle jig, its what I use to build my boxes, and I also use if for other things.
Aside from that, CdnoilRAM is right only mis enough resin to use in about 5 min. make sure you do the outside and inside of the fleece.
Also, you prolly should have angled the speaker towards the center of the truck, you get better sound that way with the crossing pattern of the speakers.
What else.... Oh remember that the fleece only stretches in 1 direction. Since your dealing with such a small piece it might not matter but if you do make something bigger its nice to know that.
How are you going to finish the pods, are they going to be painted, or wrapped? If your going to pain make sure you sand out all the resin bubble first before you bondo. It will save you tons of headaches in the future.
Remember also not to go thick with the bondo keep it as thin as possible to avoid cracking later on.
For future reference a double ring set-up would give you a cleaner look, but I'm not sure you have the room for that. It would allow the speaker to sit flush in the pod, for a built in look.
If you have any questions feel free to ask, I've been doing this stuff for about 6 years now. Most people though dont want to pay my fiberglass pricing so i stick mostly to MDF these days, but I made my fiberglass box for my Camaro.