If I wanted extra padding inside a set of grips I’d just wrap some bar tape on the bars first.
Even with hairspray, alcohol and an air compressor how are you managing to slide a grip over bar tape?
The only way it might work is if you tried to stretch track grips which are 23.something rather than 22.2mm over some very old fashioned thin cotton tape, but that would be a nightmare.
Either that or this Paul guy is using something like a CV boot installation tool.
Just buy thicker grips, ESI chunky, Supercaz XL, etc. The good think about silicone foam grips is they offer actual cushioning not just thickness. I’ve got XXL hands so I’ve always struggled with OEM medium sized grips. The only ones I’ve ever found too big were some Uberbike XL’s. They had the usual lock-on issue of being too firm to go with the size.