Since this has no lid, there's not anything to actually contain the fire. It will direct all flames and reflect heat upward, but that's all I can imagine it doing.
It might be better than having any flames shooting out to the sides and directly igniting other objects, but the flame and heat above it (especially contained in this particular fashion) is likely to be at least several feet high, enough to ignite the walls up higher and the ceiling. Battery fires are typically intense and quite hot, and unless the cans of cylindrical types rupture and blow the pack into pieces scattered around, are likely going to burn tall, wide, and hot. Cylindricals will probably vent out the cap end, "blowtorching" in that direction. Prismatics and pouches will probably burn less directionally but may rupture out the edges (particularly where terminals exit).
it's not like a gentle BBQ heat....for ebike-sized packs it's probably more like a blowtorch flame the size of a person.
If you intend for the plastic to melt once a fire happens, and let the water out, you should probably use very thin plastic bags (like crappy ziploc clones) packed around and on top of the batteries instead, so that the fire doesn't have to burn down to the plastic container, then heat it up enough to melt (with the water on the other side absorbing energy making this take longer) and let the water out.
Also consider you would need enough water or other extinguishant to cover the fire and prevent oxygen ingress to feed it, and absorb enough energy to damp the fire down, keep it from spreading, etc.
There are various posts and threads discussing possible fire containment options over the years, with assorted options including the one you note of melt thru a bag holding the battery above a pool of water (swimming pool, pond, etc) and others that chose sand in bags above the batteries so that when the bags melt thru the sand comes down onto the fire and smothers the batteries. The latter requires enough gap above the stuff to be covered so the pile building up can't block or slow down the flow of sand, and a big enoug bag of sand to guarantee completely covering the items that might be on fire (probably quite a lot of sand to make a guaranteed "mountain" over it all).
Unfortunately, or perhaps fortunately, almost none of the methods discussed here and elsewhere have ever been tested in a real emergency. If someone that has access to a bunch of trashed identical or very similar packs they could use them to test as many containment methods as possible, once a good reliable method of igniting the packs internally has been created.
