If you use the vertical space by putting in plants and branches with possibly a hide and basking spot off the ground you can essentially double the floor space in the terrarium. This enclosure would be fine for a small male garter or two.

Dekays brownsnake is also a fine option, although they are less likely to use vertical space I have seen them off the ground before. Lined snake, rough earth snake, worm snake, ringneck snake are all other small fossorial options.