From what I can tell the original Roper Plates are no longer available.
The last batch that was offered via Scud appear to be sold out.
It looks like you can get a similar item from Gutsibits which seems to be an unauthorized copy (it's not like this was patented) and we might debate the ethicality of that...
https://www.gutsibits.co.uk/pr/TheShop/index.php?f=d&Model=&q=ENA00004
At this point it looks like the Gutsibits version is the only option available
Previous aluminium plates I've made for our old race bike were up to 1.5mm thick. To be affective as a windage plate as well as a baffle you run the oil just below the plate. Because the volume of the crankcase isn't constant as it lessens it will attempt to flex the plate downwards and as it increases the reverse happens so the plate will vibrate like the skin of a drum.
The alloy plates all had a tendency to crack and disintegrate, hence the choice of stainless.
Pete