A proper cut-cab?

I'm going to do a cut-cab in my canyon and wanted a little input. Does the box always have to be Line-Xed or will fiberglass be okay? I did a search and read that I need to put supports in the box and to minus it from the air space but what makes it different from a box in a trunk, they never have any support? I would like to build my box out of MDF and then fiberglass the whole box so I can piant it to match my truck. Does the box need to be a certain distance from the cab and the rear seat? Any pros and cons would be helpful. Thanks in advance, Fred.

I have done a couple and am about to do another. I have fiberglassed one and is still in great shape after many rainstorms. I would line x or spray rubber undercoating on bottom then paint over fiberglass to match the top.  I have mine back 13 inches so i could put amp and stuff on bottom of bed.My friends is right up against the bed and sounds great also. i would recommend a tonneau cover,checkerpro has great prices. As for supports just brace box well. flexing box equals trouble,just add the volume the brace takes up back into box.

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rottenbellys 05 crew cab colorado.Pioneer dehp 880prs, kenwood excelon in all 4 doors. Bazooka 500 mono Jl 10w3 in coustom center console box.

Thanks rottenbelly. Any more opinions?