I use a couple of cardboard fruit boxes (wide but shallow, great pig height), upside down with doors cut in, when my two are outside. Cardboard is a surprisingly good insulator.
Remember too that air temperature varies around your yard, it's usually cooler at ground level on grass than at people level.
If you have good lush grass cover, evapotranspiration from the grass will help cool the area around it, more so if you add some shade. It doesn't work so well on dry grass but a couple of frozen water bottles help there. Heavy shade cloth, 90% or similar, will make a big difference too. It's more expensive but you only need enough to fold over the cage in a few layers. If you can peg it to a fence or something high enough for you too that's even better. It's a little waterproof too, for muggy humid/might rain days. Not for actual rain, but mine stay dry through the odd sunshower.
Last week we had 34 & 35 degrees celcius (93-95F) and they were outside - light breeze, 4 layers of heavy shadecloth over the grid cage, cardboard boxes and grass. Air temp at pig level was 20 degrees. In the house, 27 - 30, depending. I'd have sat in the cage with them if there was room. I don't think they'd have liked me squashing their grass though!
