While I still prefer my suggested setup using spell levels and books, if we are to go in the direction of slot restrictions
How about this, make it similar to the marina.
1- You buy and or reasearch a spell - if spells have levels only the spells you are able to craft are avaiable
- once you are done you have the ability to make that spell - Always
2 - you only get 1 spot per level of your hut - so if you have 2 spells researched but only 1 lvl - you can only load 1 spell.
3 - to switch spells - I propose either you must use a prohibitive amount of turns to change spells - a bit like the marina
- this may be an option that is to "cheap".
- or you could place a limit that you can only trade a spell 1 time every 30 days - since this cannot be altered by money or turns I see this as a good option