I found a 40 gallon tank at Petsmart with a screen lid that also has a mini-door. This mini door has been VERY useful when feeding and changing water. I moved my snake into it when he was about 2ft and he did just fine. Mt tank doesn't have side doors but I would think that sliding would be better. Swinging doors might easily be broken if swung too far. I would also go for glass. Theres no reason I prefer glass…..I just do. Also if you are going to move him to a big cage early, be sure to put plenty of decorations in it so that its more crowded and less open. He will feel more comfortable.