A keg-mash-tun false bottom (like the one shown below) costs about $60-70 around here, and while they are solid and do a good job, it was a bit on the pricey side.
I also needed some perforated stainless to use as a hopback filter, and buying even small amounts of it is difficult to find or expensive. After some searching, I found some perforated stainless on the RS Online website for about $50 inc tax. The hole-size was the same as the unused false bottom I had, but the open area was a little greater and the steel was also thinner. But without finding a better alternative, I decided to sell the false bottom and purchase a sheet of perforated stainless from RS Online - the only problem was that their minimum purchase was for 2 sheets.
The good thing about the 500x500 sheet was that I should be able to make the mash-tun false bottom, kettle filter and the two hopback filters from the single sheet. In order to fit them all from the same sheet, the kettle filter needed to be slightly smaller than the usual 30cm diameter false-bottom, but the slight decrease in size should not make much of a difference.
By measuring around the outside of the container and then gradually grinding down the edges, I was eventually able to make two round filter plates that fit exactly inside the stainless container.