How to stop marching snare from bashing around?

Hello, I'm in Grade 9 now and have just started to march with a snare drum. The slings that we have in our school just go over our shoulders and hang the snares by our knees. We do not have a leg rest, so the drum is constantly moving around. I'm holding my sticks normally but it is very hard to play like this and my legs are starting to get bruised up. Just don't suggest anything too expensive please.