I'd probably use a band of double or triple folded foil around the perimeter, sticking up beyond the top of the springform pan and folded "closed" at the ends. If the ice cream is barely soft versus melty/melting when you put on the top ice cream layer (that's what I'm thinking would be the top part anyway), and you put it back in the freezer, it should work out nicely.

Parchment paper collar on the inside of the pan

I'd go with parchment paper on the inside too, and possibly use foil to help give it structure. I'd keep the parchment on the ice cream side.

The parchment will work, but it's hard to keep smooth. If you're using the springform as a mold to stack in, rather than to go in the oven, try acetate cake collars, which are made for this purpose. They come in rolls so you can cut your own, and there are different widths, some very wide. I can't get a link in this app, but Amazon has them if you want to look at them.

Acetate ! Great idea!

But can't you smooth out the ice cream after unmolding it?

Parchment is nonstick. I think I'd just make a double or triple layer and tie it around the outside of the pan, like the photo above. The French usually know what they're doing, when it comes to culinary techniques.

I would cut a long strip of poster board, staple it together around the pan and line with parchment paper.

So many great ideas, thank you everyone. I don’t think I can get the acetate in time but that looks like a great solution. It would even help in transport. I will save that info for next time. I will try out the parchment this time.

You might need to use more than one string...

You could fold several pieces of parchment paper and stand them on end/up next to each other .... the folds shouldn't matter. the ice cream will melt soon enough