Take a look at this thread:
https://www.homebrewtalk.com/f51/sanke-fermenter-upgrade-question-182696/index4.html
I replaced the pressure cooker gasket with a large silicon o-ring. I also found some clamps that were intended to hold a camper shell on a pickup truck that I'm using to hold the lid down. I'll try to update that thread with new pics soon. I have the first batch fermenting in it now... Airlock is bubbling away, so I am pretty comfortable with the seal.
Oh, hell ya, that's the way to go! I really like the way you did that. Simple, easy to clean, and cheap. The pressure cooker gasket was a great idea.