Hi guys,
Here is a cheap and nasty solution that I came up with a year ago, right after I bought my Pioneer. It is not 100% waterproof or dustproof but I have had no problems with it. Made from 1/2" plywood, mounted with 4 bolts through the frame between the rear seats and the tailgate, and is lockable. A couple of downsides - you can't use the dump box feature with the cargo box installed, and the rear seat backs have to be released and moved slightly forward to fully open the lid (hence the holes in the sides) I carry an axe, shovel, compressor, wheel wrench, first aid kit, clevises, shackles, rope, tow straps, bungee cords, fire starter, TP, etc. It does the job for me.