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.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?