Repairing a leak in my Thermarest

Seam Grip alone should work. In the field, Tenacious Tape helps so you can sleep on it right away, but at home, where you can let it cure for a day or two, Seam Grip should be sufficient.

I’ve not used one myself, but Thermarest makes what they call a “Permanent Home Repair Kit” in addition to their field repair kit.

seam grip generously applied with a patch on top, also liberally applied and sealed off with seam grip

;)

There is a repair kit at many gear shops with patches and adhesive. When my Nemo Zor split about an inch (dog?), they sent me the same thing I already had bought at a shop.
They come with directions, and yes, clean with 91% Isopropyl alcohol found in Walmart, etc. Also deflated the pad and clamped the patch until dry (over waxed paper).
I think these kits use cyanocrylic glue, which is stronger than PU or Epoxy for this application. A slit in the side of my old inflatable Dolphin II kayak could only be repaired with the cyanocrylic.
The rubberized cyancocrylic that Roger Caffin recently posted about on MYOG might be even better, as it has more flex.
But the repair on the Zor seems to be working fine.

0.25 inch long?

How wide?  Just a slit that stays closed by itself?  Just let the matt sit flat on a table so no air will go in or out and put a little Seam Grip on to close it.  You don’t want Seam Grip flowing into the matt.  When it’s cured, put a second coat, make sure it goes 0.5 inch in every direction, really smear it with finger so it makes good contact.  Cure over night.  Rub some powdery stuff on it so it doesn’t stick to itself although that’s not really an issue in this case.  I’d let it cure for a couple days before rolling it up or putting air into it.

I’ve patched my Thermarest with Seam Grip and it’s worked for years.  Pinhole.