One of the guys on the fullsizechevy forum is doing a blow through now. He's got a bed cover so he's not as worried, but he made it out of MDF and will have it rhino lined once completed. Personally, I'd make it out of mdf, use thompson's water sealer on all the panels once they are cut and before they are installed. Recoat once all is put together, chauk all the seams, THEN have it rhino lined...hince why I won't ever do a blow through.
4" static front drop, 4-link & bags in rear, needs new paint, and a garage to live in.