Working in the Winter
If I'm planning on having this conversion done in the spring a way to work over the winter is needed. The van is too big to fit into my garage but it will fit in front of it.
Winter is here:

The view from inside the garage.

Use some furring strips, staples, a old tarp and some clamps. Block off the opening with the garage door up.

Using another old tarp and duct tape create a tunnel around the side doors. A piece of scrap plywood is placed on top of the open van doors.

Do a little cutting, put the two together and mark off where they meet.

Do some more taping.

Then install. It only takes a couple of minutes to get it fully into place
I didn't know what to call it. I was thinking of either the Human Habitrail or the Blue ET Tunnel but someone else suggest "The Airlock". I like that.

The tunnel part has some extra length so it will reach the van if I move it and not return it to the exact same spot.
The tarp doesn't provide much in the way of insulation but it does keep the wind out and with the two kerosene heaters running on their low setting the garage and the van stay reasonably warm. It comes down in no time at all and I didn't have to buy a thing. All the parts were already lying around the garage.