Hi we had a garter for 2.5 years and the very first thing we learned, the hard way I might add, is they are extremely strong and determined if they wish to escape. I am a part of another forum (strictly for garters) called
http://www.thamnophis.com/ (the latin name for garter snakes) and there you will find a wealth of info on the husbandry of garters. The people there are just at friendly and helpful as on here.
Our garter escaped three times before we got a prefabed metal screened top with clips (mind you we were using an old aquarium, not a plastic tub). We also learned that the average garter snake can lift approx. 30 lbs with their heads.
My concern for you and your garter friend would be that fabric mesh may be weak enough for the snake to rip, if it applied enough pressure, esp if you decide to nut and bolt the screen to the top, because then it will already have weak spots.
As well, no offense meant Hypan, I personally would be concerned that the heads of the bolts may be a danger for the snake. If you did not find a way to completely cover them and it was persistently trying to find a way out, it may rub it's nose or cheek on one and injury itself.
My best advice use and aquarium and a metal screen top
or join the above mentioned forum and check out some of the home made vivs they have used.
Tisha