The drag sheet on a metal bed might be fine. Might be worth trying once before investing some money.
Plastic bed liners seem to be ribbed, from what I've seen. The metal might be smoother.
My wipers squeek. Its not the blades, but the mechanism.
Stuff getting into the vents is worse.
I didn't get around to narrowing the bed to 4', and have an 8' bed. "Load lock" behind the wheel wells can defeat a lot of the LH ability, which you don't have to deal with.
Changing to a one-piece top that reaches the rear of the bed might be worthwhile. You have a big distance between the discharge chute and the sloping plywood. Looks like it could make a big mess. Cleaning up a lot of excess mess over time adds up.
Why did you choose to make a short top?
I messed with my plywood box in different ways over time. If you have the box extend to the rear, with a panel/ door on the rear, you can secure it enough to 'keep an honest man, honest", while shopping and the like. I didn't have toolboxes, so it was either keep stuff in the cab, or don't shop/ stop for lunch while working.
I imagine you will get more utility and less mess with that box being enclosed, even if its just four of those metal trailer corners
,
http://www.agrisupply.com/product.a...SKHY3VYRABXFGamv2Tuu77Ww3XUdfr5EXUaAobp8P8HAQ
two hasps, and two padlocks, if you need to remove the rear panel, rather than hinge one or two doors.