For this application you can use either 4.5mm or 6mm depending on how rigid you would like it to be. I would suggest 4.5mm will be fine, provided you have the feet as in the link you provided.
4.5mm @ 500 x 600 = $20 plus GST
6.0mm @ 500 x 600 = $25.47 plus GST.
In term of cutting the insert for it, if you have access to an Angle Grinder or fine tooth saw this will work. Alternatively you can repeatedly score the lines until you cut through, this process however could take a while.
The link referred to is this one http://www.allbrands.com/products/abp03 ... e%20Images
I'm going to give it a go. Any ideas on what I use for the support feet & how to attach them?
It's probably not worth me buying & sending on and there are a few branches around NZ for those of you further around the Country, but I could buy for those local if that's a help?