Is there a way to use a large 'telfa" patch? I think that is what they are called. They are a type of bandaid or bandage patch that have what seems like saran wrap (but it isn't) as the outer layer. That won't help w/pressure BUT it will help w/scraping, scratching, irritating the area and rash.
WHAT "other" problems are you having? IF you are having a problem w/tingling, burning, aching of the extremities (legs, feet, hands, arms) STOP taking the Flagyl IMMEDIATELY. That is PN (peripheral neuropathy), a not uncommon undesirable side effect of Flagyl and one that can cause permanent or long lasting damage.
Has you doctor approved any type of treatment for the hemmorrhoids?? Something that can be put on the telfa pad w/o being detrimental to the seton.
Have you tried some nice, hot as you can stand them, sitz baths? They can be beneficial for both a fistula in that area as well as the hemmorrhoids.
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.