I have put my 24-30" black rat snakes in 2'x4'x2' cages. They aren't scared at that size, I think they really enjoy the space. the reason neonates don't like big cages is because they're constantly thinking they'll get eaten. As they grow they don't feel that as much, so they're more willing to get out and such. which is why you'll see full grown rat and corn snakes in the wild climbing in trees along creeks you canoe down.
Personally, I think a 20L is too small for an adult corn.
An interesting idea that i'm about to do for my blue beauty snake is i'm going to make a cage out of luan doors. You know, those cheap pre-formed hollow doors you can get at home depot. I am going to get some 24" widex 80" long doors and mount them together to build a cage for my blue beauty. If you want more info let me know. I got this idea from Chris Harper over on the caging forum... I just priced the doors tonight at home depot. $21/each.
you could cut the doors and make them smaller. Would work great!
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-Chris
0.1 Licorice Stick Black Rat
1.0 Black Rat
0.1 Vietnamese Blue Beauty
1.0 Green Tree Python
0.1 Texas Bairds Rat
The educated are the few. The uneducated are the masses.
North American Rat / Corn Snake Care Sheet