How hot is the bottom of a pot of boiling water?
At a full boil, the water and steam will all be 100C, even close to the bottom, so you'll be totally fine if the bag's just in the water. (With a big bag obstructing the flow, it may be a bit less than 100C at the surface, but I don't think that's a concern for you.)

If a significant area of the bag touches the bottom, all bets are pretty much off. At that point, there's no water there to hold the temperature down. Food burns when it sticks to the bottom of pots, even with boiling liquid around elsewhere, so I wouldn't be surprised if it can reach 400F.

So I'd do like Wayfaring Stranger suggested in comments: just keep the bag off the bottom. You basically want a canning rack, but all that really matters is that it's something that's mostly open space, so water can flow around (i.e. not a plate) but solid enough to hold things up.

Some makeshift canning rack substitutes would work too:


a bunch of jar rings (for canning racks, people usually tie them together, but for a one-off you might be able to do without)
something made out of crumpled/rolled up aluminum foil
any kind of oven rack that fits in your pot
a bunch of rocks or anything else heat-safe that sinks in water