The reason why the dog lies on wood, is because it is softer than concrete or metal. A mat is a step in the right direction, but I don't think it really is a "soft spot" for the animal. (Would you like to sleep on your carpet or a welcome mat?)
I do agree straw would be too warm for summer, which is why I suggested cedar shavings instead. Fresh cedar shavings also repel fleas, ticks, flies, etc.