Oooh, hot weather isn't good for FurBabies 
Things I've tried over the past couple of years:
My pigs still like somewhere to hide so even if it was boiling , if I lifted the lid of their haybox, they buried themselves in hay (and cooked)
. DH made a screen with layers of black fly mesh to lay on the top, giving ventilation but enough dark
Old ceramic floor tiles or lino on the floor to keep cool.
Hay in a rack or in toilet roll tubes so they have enough to eat but don't sleep in it. Layers of newspaper to sit on
(I use shredded paper bedding in hot weather instead of hay to sleep in)
Juicy cucumber, watermelon, cut grass if they can;t go out, salad mixed leaves
A shade over the top of the run (old dark sheet)
Mine like to be out late (durty stop outs
)
We have two removeable windows (frame re-inforced to make it fly and fox proof) to let a breeze through and a small fan if they need it
And accept they'll be grumpy (especially my GP5 , ruby eyes don't like sun)
Check their bums twice a day (Flystrike)
Read up on heatstroke in guineas (urgent first aid needed) They go pancake flat, not just like larding on their side.
While the humans enjoy the summer, the pigs are meh