Another camper noob question - window to truck?

We had a slider window in our truck and the cat travelled back and forth.

We had a donut made of closed cell foam that was form fitted and made a nice snug fit between the two. We only had a problem once with it moving and that was while driving in gale force winds and the winds were hitting us directly on the side.

It would not be cheap to make these modifications. We ordered our truck with the slider window and our Lance came with same. I think the donut also came from Lance.