RAR–the Stowaway cantle bags are excellent for English trail riding, and hold all sorts of “stuff.” They are made of lightweight nylon, and are machine washable. I use one on long rides regularly to hold my cell, camera, two water bottles, etc. That way you can use a wither relief pad for your horse’s comfort. Just an idea.
Laytian - a member of our hunt has something like that for the dressage saddle she hunts in - I was just admiring it this morning! She uses that plus a saddle pad with pockets, so she can carry what she needs plus water for the hounds.
Thanks!