Well, it's good to hear you haven't sandwiched plywood between your aluminum wheels and your hubcaps. Get four rings of PVC pipe about 12" in diameter and maybe 2-3" wide. Notch the inner faces so they index into any slots or spokes on the aluminum wheel so they want to stay centered, then cinch them up with Zip ties. From there, maybe bent/riveted tabs with Dzus fasteners. If you get
from the street rodders you'll know it's done right.
The plywood would de-laminate in that environment.
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The gap at the back of the cab needs thought, as you say. How about the pattern is the size of the back window and as close to it as you dare. That enables replacing the glass with a rubber bellow*. Then bring the section of the topper out in a shallow curve (e.g., 12 to 1:30 on the clock viewing from the right) to an inch or two larger all round than your current pattern—such that the air in the gap is sucked like a passive Coanda nozzle. In fact that would be the perfect place to dump hot high-pressure engine air if it was mid-engined. The topper would want to be just big enough to cause the air to re-attach without adding much frontal area.
*which would block air from moving up from underneath, or sideways in a cross wind. With that feature in place, the logical place for hinges is externally, on top of the cab, like the trunk of a 1940s car. You could lift the rear a few feet and the rubber bellow would accommodate the movement.