I've done a lot of RC stuff, so..
There are LiPo bags that are so big you can fit a small child in them. ( If you're wondering, I wouldn't recommend putting children in LiPo bags. Their mothers get _mad_ )
Other things I've seen used are mortars (the kind you crush spices in, not the other one) when it's smaller stuff.
Another popular solution are old ammo-boxes.
While LiPos do burn hot and with an open flame, it's usually over quite quickly, so as long as whatever the jet of fire hits doesn't support combustion on its own, you should be fine. However, there's another aspect to it as well. They don't burn particularly clean, so there will be black flakes of soot _everywhere_, unless you contain that somehow as well.