I did the same but when I was expecting a big poo I would roll the edges towards the middle so I had a roll around the legs. You need to lay in a booster though because the rolled edges mean no extra layers are in the middle. But it contained your runny BM poo.
Big w snappis are fine but there is someone on ebay that seels them in packs of 3,6, & 12 prettyscheap & they work perfectly fine/