Think I'd go the metalwork route myself too, it wouldn't be too difficult if you had even the lightest welder. It would require a front cross member at the top, but that'll be necessary for a grill or window protector anyway. The four corner posts could simply be attached to a short piece, say maybe 2", of 1 1/2" x 1 1/2" angle, for the four corner pads, and 1 1/2" x 1 1/2" angle for the corner posts, with a full length angle along each side at the top, and one across the front. Everything could be either welded or simply bolted, excepting for the four corner pads at the bottom of each post, which will sit on the sides of the bed (which would need to be welded to the bottom of each corner post), and either be bolted or screwed through the side flange inside the bed. (if you have a hard time picturing these 4 corner pads simply take a short piece of angle and set it to the inside top of your bed and imagine it being two inches long with a post on top, two on each side, one front, and one in back, on each side, which would hold the side boards.) You could even make a quick attach rear crossmember and have yourself a ladder rack. This could all be easily made with the right tools and knowhow with very little trouble, and as easily removed as your topper is, for easily switching the two. Not sure how much fluff your bedliner adds to the equation, if it's too much you might should take it out anyway, and just go with the metal sides, instead of winding up with a 2 foot wide cargo bay.