The Aldi glass-cutting drills worked fine. My crockpot/slow-cooker now sports a pukka-looking lid with a white ceramic knob on top. I've put a load-spreading penny washer on the inside and a smear of silicone either side as a damper. I expect the washer will rust, but iron in the food's good for you, isn't it? :)
Many thanks to those who offered suggestions.