Snake Diameter Info for Hibernacula Construction

[elspru]

Hi,

In bird-houses to exclude non-target species the entrance hole is of limited size.
Similar to a species-specific bird-house, I'd like to make a snake hibernacula,

Do you know where can get information about diameters of snakes?
In particular I'd like to know the diameter of smooth green snake, eastern garter snake, and massassauga rattler.

Even if you don't have a source, perhaps you have handled these species before and thus can give an approximation.

I'm planning on making it out of a plastic bucket,
it will be mostly underground, with drainage at bottom, and rocks inside and on top.
the lid and some of the protruding sides will have entrance/exit holes drilled.

I'd like to make it a hibernacula exclusively for smooth green snake adults, and so it is ideally too narrow for both garter snakes and massassauga rattlers. Though at a minimum so it is too narrow for rattler adults (don't want rattlers in my backyard).

If there is interest I can upload blueprints and eventually photos/videos of construction and results.