They're cheap and easy to make. Just look at
this thread. You can make a cap with some EMT and some EMT couplers, hose clamps and a few screws and cover it with your material of choice. Welding the frame together is not necessary to make it work but it definately makes the frame more rigid. The welded frame makes a more stable substructure for your plywood, coroplast, or poly cover.
If you decide to make a lid you will NOT be dissapointed, I promise you!!
Swede
PS. Montana is not that far from Manitoba, if you ever are headed this way let me know and I'd be more than happy to help you out with building a lid for your F150, or you could see if Bondo has a spare kicking around. His are made for the F150's of the 05-08 era if i'm not mistaken.