Ria P from this forum has made some really good bin cages, she makes sure there is ventillation in the sides and the top, a definate must for hot days and to help keep the air flow going and the heat not building up.
I was trying to find some of her posts to link to, but couldn't find them. Hopefully Ria will see this and add some links for you. She's really good with the bin cages.
Just found one example of Ria's bin cages - have a look at the show us your cage thread
http://www.hamstercentral.com/commun...118_083016.jpg