I contact print mine with two stuck into each 35mm sleeve. I use a tiny bit of tape to hold them side by side.
I have actually found that they get very close to being flat if stored well weighted this way.

Then you can actually project them while still together in a 35mm neg carrier. I have masking bladed thing to put under the carrier that helps to isolate the frame. I use a 50mm lens, (until a shorter lens can be found, and a tall head height