Cellars (or "basements") are standbyes for many North American brewers and can provide a sufficient degree of temperature control for certain brews and yeast strains.
A common practice there is to place the fermenter in a tub of water, at least for the active phase of fermentation, which generates a lot of heat. If you do that, the water conducts the heat away and the wort temperature does not rise too far above ambient, depending on water depth and maybe changing the water or adding ice to it at peak activity.
Your summer cellar temp is in the high end of recommended ranges for most ale yeasts and far above lager range. So see if you can use a tub without contaminating anything. In any event you might see if you can pitch a bit lower in the yeast's recommended range.
There is some discussion here on the kit yeasts at 20 and above. Also, some brewers have used S-189 for lagers at 18-19 and liked the results, but I'm sceptical. More certainly, you can get "California Common" lager strains that produce good brews at 20 or so (as in Anchor Steam Beer if you can find it), but only in liquid form that I know of.
