Hi are you able to remove the seat from the tree if so patch it from the under side of the seat much neater fix, looking at seat the i would put a larger patch than just for the tear because looking at the area around the tear, looks fragile as well that would help strengthen that area too(saves another repair in near future) glue it onto the seat the stitch onto the skirt trim excess with skirt shave/french edger, just make sure you have allowed enough so that you can stretch it back onto the tree hopefully you will not have to stitch anymore stretching back onto the tree should hold it in place as Ron has said not a quick or cheap option but quicker and cheaper than replacing the hole seat and a much better fix than a patch on top also a good learning curve for you. The leather i would use would be pigskin or kangaroo both strong and light weight.
Hope this helps
JCUK
Hi nice work, its been a long time since i made one i believe the chain fitting is a 1/18'' or 1/14'' pretty sure 1/18''
Hope this helps
JCUK