I know water-beds are cheesy, but I had one in highschool (class of '77), and they will keep you suprisingly cool in summer.
Box them in and install a plastic liner, it is only a matter of time before the bladder springs a leak that must be patched.
If you use cold water in a garden or for some other non-pressurized reason, you could pass cold water through the water-bed on its way to the garden (or to a holding tank that is lower than the bed)
Also, in winter, you can set a flat electric heating panel under the bladder and very cheaply stay warm all night.
Very easy to drain, unbox, and set-up a conventional mattress for the half-year you don't need cooling.
Also, perhaps set out plates of dessicant in the closed room (RV enthusiasts do this) to dry the air, this will make the ceiling fan and skin-sweat provide better cooling. Just a thought...