Last night I had time for a quick play with the steel paper sheet I ordered online.
As you can see from the photo, it also passes the "minis upside down" test, which I'm very happy about.
Plus no more issues with pole alignment!
The non magnetised sheet is more expensive than the magentised, but in theory I can make a number of steel bases that can then match up against magnetic based figures from a range of periods.
I'm pretty excited about this...