Help?

[snake momma]

So I recently got a 20 gallon tall tank that has driftwood, fake plants/vines, hides and a small rock water bowl. My previous home for my 2 garters was 55 gallons and I can't help but wish I had something bigger. ( the 55 gal was a tub and was temporary), so...there is a hole in the bk of the 20 gal that currently has a stopper in it but I was wondering if it was possible to make an extension. I really wanted to make a larger water area Pls let me know if u think thos would work, any ideas, or wether I should make the extension full of water. Thanks. Oh, I have two, young garter snakes