I would say you are n the right track already. Very hot water will definitely help. Don't under-estimate the effect that some lube will provide. I use some dish washing liquid. Just squeeze a little inside the grip, spread it with your finger and they slip right on. After a day or so they don't slide any more. But I would say the hot water should get you started. That's how mouth guards are shaped, and they are some pretty firm stuff.
The only problem I see is that if the grips are designed for 7/8th bars they might split on 1" bars.