I can pass on an alternate method from an earlier post of mine:
My Unimog camper, Camper Mog (built in the Netherlands in 1995), is done differently from many. Lots of campers use a flexible bellows between cab and cabin, but sometimes it's hard to keep the bellows in place and, besides, with the big passthrough on Camper Mog (about 5 feet tall by 2+ feet wide), the bellows would be really big and expensive. Instead, the rear of the cab and the front of the cabin each have a flange around the passthrough that extends about six inches, but the flange on the cab is a little smaller in height and width than the one on the cabin (or it could be vice-versa; can't remember for sure and it's not here to check).
The smaller flange fits inside the bigger flange with enough clearance that the two can twist independently, but the gap is small enough that water can't leak in. (And there's a bit of weatherstripping involved, too.) This seems a clever approach and it's proven a trouble-free way to do it