What kind of fermentation chamber are we talking about? Chest freezer? Upright freezer? SOFC?
Have you ruled out corny kegs? They're (usually) only 5 gals, so using them for primaries on 5-gal batches might get hairy... But, they're (relatively) cheap, and could likely stack pretty well, assuming you get the type with rubber handles on top, not the single-handle all-stainless versions.