Angled crib mattress

I currently have the crib mattress angled up at one end to help with my LO's reflux. I have always put two rolled up muslin blankets on either side of LO (at armpit level, never at face/head) as he slept to keep him in place. The hospital did this with their angled bassinet. Even though his crib is currently in my room within earshot, I get anxiety about SIDS having the blankets in there. Well, I took them out last night and he woke up crying at 2am (he usually isn't up before 6) and was completely upside down in the crib! His head was down lower than his feet... Terrible for reflux/gas! So I'm wondering how all those who angle their mattresses keep their LO from sliding down and moving around like that? Thanks!