Cantle bag no. And there are some much cheaper alternatives to the english made hunting sandwich case. Small, leather, and about the same size. Shop around for facsimiles!! .
But as a diabetic; I never needed one. Granola bar during the hunt got me through and try to time your pre hunt meal to anticipate this. I found I had to eat on my way driving to the meet. Most of all stay hydrated to keep BS even!!
I don't foxhunt, but I do ride all morning or all day, moving cattle and such on a ranch. I have fibromyalgia and get migraines, and if my blood sugar and electrolytes aren't OK, I get bad, fast.
I have a really hard time eating in the morning, but I can drink fluids. I learned that drinking a quart of gatorade during the trailer ride up is one of the best things I can do to feel good for my ride.
Saddlebags are no problem for me, so I have a couple of snacks in them. But I could easily carry them in pockets or a sandwich case.
Welcome to fox hunting! Hope that it works out for you!
I fox hunt with a gal who lives with hypoglycemia. She carries a couple bars in her jacket and a little liquid in her sandwich case and she has never had a problem. She uses a case like the one altjqeger posted a link to. Bartville Harness offers a traditional looking sandwich case for about $40 more than the one linked. So then it comes down to the individual hunt- do they require the bag be traditional looking or will any conservative leather side bag do?? Maybe find that out first from the hunt then make the purchase.