Hello bac,
re sheet sizing... If you use the 90mm glasses then 125mm square is enough to work with =3cm for gasket area, and rods in the corners.
For the 115mm glasses suggest 150mm square sheet on the same basis although the rods will be thicker on this version.
There's a number of vendors who sell pre cut square sheet in suitable sizes avoiding the cost and effort of cutting sheet... here's one in Aussie
https://www.aemetal.com.au/Webstore/c-148-bshco0p0000.aspx
On the issue of thickness. I used 1.2mm sheet for the 90mm glass model. For the larger 115mm glass version I use the 2.0mm sheet squares.
Just on the strength issue here...the sheets should only ever have soft soldered downcomers and minimal heat used...this preserves the sheet "stiffness" ie don't ever anneal.
The stiffness is a real bonus when building and keeping a nice level sheet when tensioning up.
Brendan so happy with the performance of those gaskets as I guess you would be to...they just last and there's no issues with them.
Hope this helps!
bt1