I use IV bags, at my farrier’s recommendation. They’re FREE and plentiful from a local equine hospital. They’re pretty tough (average life span 15+ soakings), but if one pops a hole it’s no big deal. You don’t have to use too much water, and most horses don’t mind shuffling around a stall in them.
I always put the bag on the foot with the seam pointed north-south, then slowly pour in some water. Chances are the horse will move, possibly spilling the bag so don’t add the magic potions until the horse is comfortable and still.
I usually hold the bag around the pastern and pick up the foot so the horse feels the weight of it; then put the foot down, add Clean Trax (or whatever) and top off with a bit more water. A snug (but NOT tight or constricting!!) ring of duct tape around the top fetlock, and a looser ring below (around pastern) keeps the bag on. Duct tape sticks REMARKABLY well to the thick vinyl bag… sometimes it’s even re-usable! Once the bag is on, squeeze the water bag to mix the contents. I pick up the foot again, slosh it around once again reminding the horse that there is something on his foot.
I use about half a bottle of Clean Trax per treatment, since there’s quite less than a gallon of water in there. I also use the IV bags for iodine/epsom salt abscess soaks, and for icing feet after xc. My farrier recommended keeping a separate dry bag for the clean trax “gaseous phase”, but sometimes I’m lazy and I just empty the soak bag and tape it back on despite it’s inner dampness (I’ve not noticed much impact on Clean Trax effectiveness).
It is VERY IMPORTANT to acclimate the horse to the weight and sloshing of the bag on his foot before you walk off and leave him alone. I usually soak in a stall-- bedding cushions and protects the bag, and an enclosed environment is safer. Many of them have a minor freak-out when they first discover a monster stuck to their leg. I always introduce it slowly, picking up the foot carefully to avoid surprise. I may encourage the horse to take a few steps on his own, to prove to himself that it isn’t a life-threatening experience. Once I’m sure they don’t care anymore, I’ll give them a flake of hay and leave them unsupervised. It beats the heck out of babysitting a foot in a bucket!!