Worth contacting Thermorest / Cascade designs as they have a repair service plus their mats have a lifetime warranty. Failing that they'll give good advice on home repairs.
Exped do the same - I ripped a 3" slit on a root on a 4 year old well used mat. Posted it off and they did a really solid repair and posted it back all FOC.
"My God, Ponsonby, I'm two-thirds of the way to the grave and what have I done?" - RIP
"At least you got some stories" - James Acaster
"A little nonsense now and then is relished by the wisest men" - WW