Instead of “travel plugs” I used the frost king foam gasket material like was supplied for the door and hatch gaskets. I installed the front vents using the goofy white rain guards and I needed a layer of 7/16 thick plus a layer of 5/16 thick to get a seal when screwed on fairly tight. On the rear I didn’t use the rain guards so it took two layers of 7/16 to seal. I can unscrew them enough for air flow without having to totally remove them.
BTW, the same outfit that I linked to sells a set of 4 vents for just under $40/free shipping.