I use glassine paper myself. It is pH neutral so is safe for long term storage. It is also pretty economical when you buy it in a large roll. I use the Borden & Riley brand which I buy at a local art supply store (in Seattle), but it may also be found online.
I would assume kraft paper is acidic and may cause harm over longer time periods.
I don't have any direct experience with MicroChamber paper, but it seems like it should be suitable too. It is more expensive, however.