The iplosion of a steel vessel is not like the implosion of a TV-tube. I have seen a vessel (several cubic meters) that was full of steam when Mr Murphy let in cold water. It looked like what you can do to a beer can with your hands but there was no damage to anything or anybody else. We always had our autoclaves designed to withstand a mishap like that. They were in principle, long tubes 7-8' diameter and 3/4-1" wall thickness. It was enough to put rings on the outside to make sure they did not implode or (rather squeeze flat). I should think that those pressure cookers, I have seen, would not be subject to any damage since the cylinder part is so short. The damage type is buckling and the "gables" stabilize the cylinder and prevent that. However I would not use a pressure cooker.

In my part of the world a standard steel tube of appropriate diametre plus two gables of steel plate with gaskets of rubber sheet would fit the purpose. No welding needed or recommended. It is possible to find tube specified to withstand 100% vacuum. I would attach a nipple to the tube. (drill and thread), Lay one steel sheet on a table, A sheet of rubber on top, put the tube on the sheet then next sheet and the second plate on the tube. The vacuum will keep the bits annd pieces together. Obviously the tube should be cut cleanly in a lathe or other machine. I am not talking about cast Iron sewer pipe but pipe for pressurized applications. A supplier of steel tubes to the industry would be able to supply the tube cut to appropriate length. If you need a window , cut a hole in the upper sheet and put a sturdy piece of Plexiglass with another piece of rubber on top. 

Thanks Gote! I'll be looking for some steel tube next time I'm at my metal supplier. For the size I need, I'd think 1/4" wall thickness would be plenty sufficient.