The only sure fire way is to make a small feeding pen and shut the OSB in it while it eats.
You could probably knock up something quite easily with a couple of pallets or hurdles - you'll be there while they're eating anyway so can keep an eye. I'd also scatter the kk's feed over the grass - it'll take them longer to eat it and force them to move more. Theoretically, if the kk are over about 8 months (and for the freezer, not breeding) and have plenty of grass, they shouldn't need any hard feed at all - I've always just given them a handful anyway (but just one handful!) to keep them quiet
HTH