First, that is only a "Speidel" like tank. Probably made in China. Not an actual Speidel tank (made in Germany). Some clear signs: The Official Speidel logo is different. No real Speidel tank has a sloppy casing seam down the center like that one has. The volume is marked differently on the official tanks. And a real Speidel tank is much thicker in construction, so would never "curl" as you experienced. I know. I have many real Speidel tanks.
Sadly, such knock offs are not always illegal in the USA.
That being said.... Your mod was well done, and appropriate for this tank. But.... you need not have needed to do it if you got a real Speidel tank (but they cost more). Hope this helps.