You seem smart enough, so I’ll add my opinion…
Be careful with cardboard spools, even a dryer will dry out & flake causing dust to get all over the filament.
Drying like you have it could make the spools warp download, this is why filament dryers have the spool standing up…also so you can print from them.
Myself, I would make the bottom as flat as possible & the same width of the spool…JMO