I used 3/4 inch plywood. I cut 4 pieces 27" length, 2 of them were 6" high and the other 2 were 5" high. The box height had to be a maximum of 5" or else it would not fit under the dash. I then cut 2 squares and attached to the box ends, using screws to attach all pieces. The speakers came with a template I used to cut the holes. All was done with a jig saw. I made a small hole on the top towards the back and ran the speaker wire through it.
I just picked up matching carpet and will be wrapping up the box, should blend right in. I'll send you pics once I get around to doing it.
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