Ooh Nic, please be careful about adding to a portacot mattress. I read this horrible article today about this little boy who suffocated because his parents added a mattress on top of the ortacot mattress. He got stuck between the mattress & the side of the cot.
I won't use one that isn't specially designed for a portacot for that very reason. but you can get them made up so they are a snug fit.. I do know the baby shop here said they get foam or something similar cut it to the right size and there is no risk of suffocation.. But we opted not to do it as we don't use it enough