I got my correx on ebay but it was quite expensive. I bought about 8 sheets of A1 size I think and it cost about £25!
Or you could ask your local supermarket for large cardboard boxes and use the sides of them clipped together with bulldog clips. I do like the correx as stays firm and can't be chewed like cardboard.
Zooplus sell this wood one for about the same price. It's 30cm high so should be ok if you keep toys away from edges
A reviewer has used it for a Syrian.
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The castle one is cheaper but it's only 23cm up to the battlements and 30cm total height which I reckon a syrian could climb over
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You could make something bigger with correx I guess but it does seem expensive stuff these days. They seem to be charging about £20 for 5 sheets most places
https://www.amazon.co.uk/840x600mm-s...=2mm+correx+A1
Or about £35 for 10 sheets which seems expensive.