Building a hibernaculum is something Sheila and I have talked about. We have talked about making one at the local dump - so the bullsnakes could control the rats, and we have also talked about making one in the basement of the zoo where people could look at bullsnakes brumating through plexiglass.
The best garter snake hibernaculi that I've seen out in this country, are old farmhouse basements that have been collapsed. Abandoned empty cisterns, and even the bottom of old abandoned farm outhouses (that haven't been used since the 40's) also make good homes for garter snakes.
If then can wait until summer, take house off of foundation, wreck foundation, and build new house away from original foundation - they could probably make great hibernacula. Personally I'd throw a bunch of pipes and large rocks in the old basement hole, and then pile some dirt on one side of it.
Garter snakes find their way back home, I think a combination of pherimones and natural honing device that we do not understand - lets these snakes know where home is.
I'd love to have a big snake den in my backyard.
Wait a minute, I guess I do. Rattlesnakes and bullsnakes den in the thousands 12 kms from our house. Out here 12 kms takes about 8 minutes to get to.
I'm sure that if he so chosed, he could make an elaborate set of tunnels and caves with different sizes of pvc pipe - but I'd go for the natural aspect.
Could he leave the old house there for the snakes, and build his new one somewhere else in the yard? That way, the snakes could use the house overwinter.
But I do remember seeing plans for snake dens at one time. As far as I remember, it was a lot of large concrete blocks piled up askew, and then partially covered in dirt.
Anyways, I'm glad to hear that people are going to try to help the snakes - that is great.
Ryan and Sheila