I would recommend 4 things to minimise the risk of a LiPO fire
1. Don't crash
2. If you must crash - do it gently. Avoid flying 'fast' planes when the ground fire risk is greatest.
3. Build the battery box in such a way that the LiPo has a 'crumple zone' ahead of it (as big as possible) so that in a sudden impact the g loading on the battery is reduced as structure breaks up.
4. Avoid surrounding the LiPo with anything that might pierce it. In a realyl severe crash as a conventional close fitting play battery box splinters it is quite capable of piercing a soft LiPo case.
Because a foam plane gives and crumples a LiPo is much less likely to get damaged badly enough to cause a fire.
A LiPo is surprisingly tough. I had one fall out of a plane from 150 feet onto grass with no damage. It is still in use!