Well, I've never done that, but I would think a bag that was porous would work better than a plastic one to let the leaves breath and to make sure no moisture develops. Also I would put them up off of the floor of the cellar as well.

Freezer?

Maybe you should have a big ironing party with all your friends and iron them between wax paper. Then you can store them stacked up in sheets of paper.
If you put the in bags you will end up with brown curled up leaves I think. So at least iron a hundred or so to have some color.
Kathy

You could dip the leaves in wax to preserve them.

I've stored leaves through the winter by spreading a tarp in the garage, spreading the leaves out on them, and letting them thoroughly dry....took about a week's time during a fall period with moderate humidity. I fluffed and turned them a few times during the drying period. Then I rebagged into paper leaf bags and stored them in the garage for use when the new garden area was ready. They came through with no mold or decomposition and could be spread without the sinus headache I get when stirring up moldy leaves.

Of course the brilliant fall colors fade as the leaves dry, but muted shades of yellow, red, and brown remain.....depending on what leaves you start with of course. Not sure just what you want the leaves to look like for your project.....but drying will at least keep them from getting moldy and stinky.