I installed a bedliner that I got from a Datsun 521 truck. It is not made for a Datsun though and was previously trimmed. I had to trim it a bit more for my setup.

My truck looks so much better now with a black bedliner and this solves my problem with the single-walled bed sides. No pictures yet! I'll get some tonight when I get home.

Thanks dudes!

I got that from a 521 parts truck I have strategicaly placed in my back yard. The bed liner is the correct width but is a little too tall and long for the Datsun. The front and back are cut out. In the 521 it had top rails that set over the edge of the bed sides to protect them. I cut it off flush with the bed so I can build a hard tonneau cover later on. On my truck I just painted the front of the metal bed black and the bed liner only covers the floor and sides of the bed. I know water may get under it and cause rust but that's ok with me. If I get a cover made in a year or so that should keep all water out of the bed from that point on.