My sister did this with her DS, until they moved into their new home when he was 7 months old. He was an excellent sleeper so it didn't cause any issues with him.
I agree that it could hurt your back, unless you get a portacot with the bassinet insert. Ours has one and it was fantastic when we travelled with DS.
Please don't add an extra mattress to it, babies are safest and perfectly comfortable with the 'mattress' that comes with the portacot. If you add an extra mattress there is a risk that your baby could roll between the mattress and the side of the cot and suffocate.