Hi,
I have used two types of muzzles for my part draft/paint gelding successfully. I will share what has worked for managing their health successfully.
Two types we use successfully- Best friend grazing muzzles or Green Guard (www.harmanyequine.com) from England. Both take some figuring out and constant monitoring/adjusting/repairing, but are worth the peace of mind in knowing my horse won't suffer from more serious health concerns from the sugar overload and the sheer amount he can eat.
We can go through 3 muzzles per year, (best friend type) as my gelding needs them 24/7 and the hole in the bottom gets chewed through and enlarged too much. The only time we don't use one is from mid or end of november thru feb1 or mar1 depending on how the temps drop. If nights go below 32, I take it off and put back on during day / higher temps in winter. He successfully eats grass, long grass and hay. They learn to eat quickly. It is a bit frustrating the first few days to a week or so for both you and your horse while you adjust, find lost muzzles and figure out teh adjustments and what works.
I use 'Gorilla' tape to repair and adjust. Just cut the tape in half for a more narrow pc and make long pcs. of 12 inches or more to wrap around and between the holes to repair the muzzle in any number of areas. This tape really holds. It can only be removed with scissors and does not harden or crack with weather. Even my horse cannot chew it off.
Another must for us is to use real fleece halter wraps. I wrap all the straps that come into contact with his face to prevent the rubbing off of hair. Sometimes I have to cut special holes and weave extra cord thru a piece to ensure it stays on after the velcro wears. If you keep two muzzles on hand, you will always have a spare when one is lost or needing repairs. The real fleece washes my hand very well. Again, I keep an extra set so I can replace a lost piece until I find in in the field.
The nose area is especially important to pad with real fleece as the weight of the muzzle pulls the halter down. I cut a hole in the center of the nose piece to allow for the muzzle strap and to keep the nose piece attached well. It tends to fall off more easily.
Lastly, I keep Desitin on hand and put on his muzzle for the first two weeks or so until his skin gets suited to wearing the muzzle. I also apply whenever he appears to be getting a rub on the nose, before a sore appears. He doesn't like the smell too much, but it has zinc and heals the skin rapidly and does not sting. I also use in on his white blaze to prevent sunburn all summer. It can be rubbed in at night, so it is not so messy when you want to ride during the day.
Also, one last trick - my friends braid their horses manes around the top of the halter, and put a second breakaway halter over the muzzle for those escape artist horses.
It was hard for me to get used the muzzle wearing and all the hiccups, but it is much more humane than overweight issues and allows the horse to walk in the pasture all day alongside his mates.
Good luck.