Can I store my carousel boxes on their side like a library book or do I have to lay them flat? My concern is sideways, the slides may buckle.
Thorough this year, I will digitize them. However I want to keep the originals.
There is very little danger to storing slides in the carousel vertically or horizontally in regards to bending or warping of the slides themselves.
You should, however, consider the materials of the slide mounts and the boxes the carousels are stored in as these can greatly affect the longevity of your images.
If you want to store these long-term, it may be worth the investment in archival slide mounts to replace any cardboard ones along with either boxes or binders with sleeves. Look for acid free materials specifically made for archival storage.
Assuming that, once you've digitized the collection, you will no longer be using the carousels for display purposes, consider that eliminating the carousels altogether reduces the space needed by 75% (or more) and depending on the model they can be sold for $5-$15 each.