I have an 11 year old gaited gelding. I moved him to the barn where he is now 3 years ago because I wanted him on pasture board, and it has been a 100% improvement in mental and physical health, but because of personal issues in my life, I’ve been forced into being a caretaker for the past 2 years because of 96 year old mother’s 2 bad falls and husband’s heart and kidney failure, he’s had too much time off and has put on too much weight. He gets excellent care, bodyworker every 5 weeks, chiropractor-accupuncture every 3 months, etc., farrier, teeth, etc., he’s not neglected, but what he needs the most is regular routine of work and he’s not getting it. Where I am in east Tennessee there is not an abundance of qualified experienced people to come and work him under saddle. To make a long story short his practitioners, especially chiro and osteopath, both have told me “he’s over weight and you’ve got to do something about it”. There is no dry lot and won’t be where I board. My only option is a grazing muzzle - I can’t see him keeping it on without hurting himself trying to get it off based on how he does fly masks and boots - ANY AND ALL comments are appreciated!
What if instead of looking for a trainer or exercise rider type, you looked for a partial lease? Even a free lease. X rides per week. You might be able to find someone experienced who may be interested.
Believe me, I know how hard it is to find a barn with a dry lot that is also offering good care, but check around. Might be worth moving to a new barn if you can find a dry lot.
I used grazing muzzles with leather break-away poll straps. It worked somewhat, I don’t think my gelding was able to get them off himself, but he DID get his buddies (even over the fence!) to take them off for him. 
I ended up making sure I had at least two grazing muzzles available and tied a short bit of red yarn to it so I could find it when walking the pasture when he came in without it. 
Eventually, I wound up moving to a barn with dry lot paddocks. So try it with your horse and see how it goes. It might work if he can’t convince his friends to oblige him in taking it off.
The flexible filly grazing muzzle is fairly well tolerated and offers adjustability. My pony wears his with zero issue and hates all the other ones.
Greenguard makes a lovely one with a halter that’s difficult for them to get off.
We have five horses in these. Agreed they are well tolerated and hold up well. If one orders directly from thin line, there is an option for two different sized holes. We use colored duct tape on the muzzles of the ones who occasionally get them off.
I will third the recommendation of the Flexible fillies muzzle. It’s easy enough to use them with a breakaway halter.
My favorite thing about these muzzles is that they are much better ventilated and less hot for horses than some of the other popular options.
What a great idea!
Flexible Filly and Greenguard are both excellent muzzles, and they each have their pros and cons. I have one horse in a Greenguard and another in the Flexible Filly. Another brand I would consider is the Tough One Easy Breathe. It’s cheaper than the others, and for this reason might be a good muzzle to use while you’re learning more about muzzles.
It’s generally not recommended to have the horse wear a muzzle 24 hours a day, although some people do that. I’ve read that some manufacturers recommend no more than 12 hours a day, and my vet said up to 18 hours a day. Ask your vet how long your horse should wear one. Also, if a horse that wears a muzzle all day is then turned out at night without a muzzle he’s very likely to eat more to make up for what he couldn’t eat during the day (ask me how I know). Do you have a place to confine your horse when he’s not wearing the muzzle? If not, could you create a space with temporary fencing or corral panels?
If the muzzle is attached to a breakaway halter your horse would be very unlikely to hurt himself if he snags it on something. Still, somebody would need to check on him at least once a day to make sure everything is ok.
Good luck with this. You’re doing the right thing, because your horse is at the age where the risk of laminitis goes up.
Another one I like is the SweetNet muzzle. My mare has tried the GG, FF, and one of the tough one types. I like the FF the best, but she figured out how to eat out of the side. I then tried the GG, which I think slowed her down the most, but she figured out how to take it off (even with the GG halter and the extra neck strap!). She tolerates the SweetNet. My only criticism of that is I think it’s the least restrictive of the three. It comes in multiple hole sizes - I have the 2 inch hole. I’ve thought about going down a half inch, but right now my mare is on a dry lot. 
