I am my 3rd season burning a Summit. I have replaced the gasket every cleaning. Most will find that will be the case for them also.
The gaskets while relatively inexpensive, just do not hold up. I got tired of ordering & replacing them, so I made my own out of standard 3/8" stove rope gasket.
Here is a post with a picture of the one I made in it.
Custom rope gasket
BG, to this point, the stove still gets secondary back at the rear of the baffle(single gasket, doubled gasket, rope gasket). If its getting that with the 3/8" rope gasket I used to make the baffle gasket, I don't think it will stop.
I also did the double up of the stock one. I noticed on my baffle, the surface that mates with the vertical channel, has a minor imperfection and is not perfectly flat in one corner.
At this point, I don't know how well the rope gasket is holding up, I won't know until next cleaning, which will most likely be fall, unless we get the snow melted off the roof & a decent day.
If it works out and stays intact, I'd be happy to make and send any other Summit or T6 owners one of my version of the baffle gasket, or explain to them how to make their own. They are simple to make, and if it doesn't disintegrate, it will eliminate the constant need to change them. I found I don't mind the secondary burn off in the back. There is no lack of secondary in front & bottom, and it seems to help the wood burn more evenly.