Gelatin filters

How do you use these? I see a couple of gel filter mounts for Cokin and Lee filters, any other ways if you don't have one of those systems?

For reference, I purchased a Fuji 10-stop ND gel filter (plan is to play with it before buying a larger filter.) I thought the Sigma 12-24 had a gel slot in the rear, but apparently that's only the 1st generation!

some of the canon wide angle lens have square cut out in the rear for use of gels,but not all canon lens line up have it,they also make a gel filter holder that fits on the treads of any of the canon line up of lenses,its a square gel filter holder comes in two sizes,but there not cheep !