I would not bother with free slots these days and they were my a good part of my bread and butter in past years. Fro example, not too many had the 3d slots back in 2010, google it now.
Some are a bit of a pain to maintain like the igt slots and you should really make sub-campaigns for each slot or game.
So many casinos have free play slots on their sites these days. I’m a little suspicious of tracking when you send a player to a casino landing page, they start playing the free games and decide to register through the game.