Spent a couple hours last night building a bed cover, since they're calling for snow this weekend in Maine and I won't have a garage to put my truck in for a couple more weeks I decided it was worth it to build something so I wouldn't have to shovel out my bed.
I built a frame out of 2x3 studs that follows the dimensions of the inside of the top of the bedrails, and then screwed it to a sheet of MDF. With my 5.5' bed and tool box I have in there, the 4' wide sheet of MDF was the perfect width; I just had to cut it to length.
The frame slides under the 'lip' on the toolbox, and when you slam the tailgate shut, it sort of wedges it into place. I took a couple eye bolts and screwed them into the frame so I could use tie downs to strap it down to the hooks in the bed but It's in there tight enough that it probably isn't necessary.
I ran a router around the top of the MDF to make it look a little more finished.
I used bedliner paint to finish it and it came out... less than satisfactory. Next time I'll buy the roll on stuff instead of the spray on stuff.
Looks great! I thought about marine grade plywood but wasn't sure how it'd look after I ran a router along the edges. For the money I have invested in it, if it doesn't work out I'll just build another one.