My trailer build
It has a gap of 1/4 inch at the hinge but then tapers down to nothing at the tailgate. I need to install some sort of bumper at the rear to keep it from crushing the new seal i put on.
You can get rubber plugs that would give you the gap you need. Our experience is those get crushed as well over time. We now use natural felt which seems to hold up much better.
You can get bulb seal with a sticky back, comes in different bulb sizes and widths. Mc Master Carr has a good selection.
If you want the bulb and felt from AT we can also supply.
I actually found a pretty decent automotive weatherstripping at Lowes. HD doesn't have it.