I wouldn't trust the slow feeder with my drafts. They destroy everything to get that last crumb of hay. Anything you can do to limit how much hay they can get at one time will help. A regular hay rack with a hog panel inside of it to make small opening, wrapping the roundbale with snowfence or a hockey net... Look around and see what you can make with what you have around. Buying hay with a lower RFV (relative feed value) will have less calories. Don't buy the prettiest stuff that they inhale, buy the mature stalkier stuff that most horse people pass on thinking it's cow hay. Still needs to be clean and moldfree. Straw can be an acceptable chew hay provided it doesn't have the grains, is moldfree, is bright... It's not going to provide much nutrition but it will keep them busy and warm.