Use Tris-EDTA (TE) buffer. Tris inhibits acid-catalyzed hydrolysis, while EDTA keeps it sterile and inhibits hydrolysis by enzymes, which are all metal-dependent. If you have the freedom to refrigerate, do that, and make it as cold as possible. Liquid nitrogen is very safe, and -80C too.
If you can't refrigerate, the clonesaver paper is probably your best bet. We have good experiences with TE + regular silica filter paper (GF/F). You can spot on paper and keep them in the freezer as a backup, to guard against freezer failure. Make sure everyone knows where the papers are kept, so they can grab them first when the building is burning.