The boots option is out at this time as I am (rightly or wrongly) led to believe that they prevent heat loss? I found it hard to believe but that was from a SAR dog team
Even in the hottest weather my dog had no problems - the boots did pong a bit in warm weather but they did not seem to cause him any problems once he got used to wearing them. A quick wash in some soapy water and the boots were good to go again.
I suppose the longest he would have been wearing them would have been about 2 hours max. He never wore them in the house and on nice soft ground (grass etc) I'd take them off. They were only really needed for pavements and hard surfaces.
I can recommend this brand - once he was wearing one on each paw I had to buy about two a year as replacements for the ones which wore out (he used to drag the toes sometimes and wore through the rubber on the toe of the boot.
I used to get them from this company in Huddersfield. Nice people to deal with and very fast with the shipping. usually arrived in 1-2 days. They will send you a sizing chart to make sure you get the right size for your dog.
The only problem putting them on a greyhound - if the dog is off the lead and spots a rabbit or other small chaseable furry thing. When mine took off from a standing start it was like watching a sabot round fired from a tank - the dog would make a straight line for the target while the boots flew off in all directions
Edited to add: the Therapaw boots have a vent and breathable material to allow the paws to sweat.