Wife noticed birds making a nest in the 1 post hole in her RAM1500 (with the black plastic bed rail top). She put sticks in the hole to deter, but they were adamant about building in there.
Took some quick measurements, whipped up this plug/cover in Fusion360, and in 1.5 hours had a plug.
Its snaps in loosely but enough it wont pop back out without force.
Within 10 minutes of using a coat hanger to dig out about 3-4 cm3 of nest material and snapping the plug in, the birds were pecking at it trying to get back in :-) (but they couldnt)
Printing 3 more for the other hole in her truck and both in my 2500.

The STL Preview above is being weird, showing a slot in the top around the inside edge.
It does NOT print like that, if you click the 3d view icon and spin the model around, you will see the top is flat.

Prusa

i3 MK3 MMU 2

Doesn't Matter

Doesn't Matter

.10mm

70%

Print with top flat side down, didnt use any supports, it works.