My 4 month old is getting very wriggly in the crib and I think it might soon be time to move him out of it into his cot. I have a second hand cot which only has two base positions and the highest one still looks quite low. The side lowers but only a bit. I am a bit concerned about this as I normally put him down asleep (which I know I need to work on) and my back tends to play up if I put too much strain on it. Do you think it would be ok to put two mattresses in it to bring the baby higher, at least until he can sit up?