I think Curleydave has the right idea, determine how much space you need, rent a U-Haul box van with a car dolly. Load your pickup with items unaffected by weather or tarp items in the pickup bed, load everything else into the van and head South. You are going to spend a lot of time and money on fabrication of something that may or may not serve the purpose then be stuck with a big wooden box after the move. If your pickup has a towing package you might want to consider finding a used 6x12 or 14 used tandem axle box trailer, make your move then sell it. I still think the box van and dolly rental is the safest and easiest method. Good luck.