I keep eggs that I may incubate in the utility room (between kitchen and garage), no heating, in an ordinary egg box, seems to work well, I'm sure people in the know have a much more sophisticated system, but this is all I have.
Five or six days seems OK, i.e. just kept some ebay Legbar eggs after posting, to incubate with my Croad eggs, as I wanted to do just one incubator lot, so ebay Legbar eggs are quite a few days old, and all six Legbar eggs are fertile and incubating at the moment.