Do *not* put another mattress into a travelcot or portacot as it's a known suffocation risk. The recommendation is to place no additional mattresses or padding inside cots and especially portacots. I agree with Traveller, much better to pick up a second hand cot. Check out the Baby and Kids Market (run regularly in Belmont, South Fremantle and Warwick). We got a lot of very good stuff for very cheap prices at these markets. .
Where about in Belmont? Is it the 2nd hand market on Sunday at the car park of Belmont forum?