I have a very similar challenge, so I will add it to this thread if no-one objects. We have 14, one-week old chicks, who are currently in a spare bedroom in a large Rubbermaid tote. I'd like to move them out to a larger brooder (6'x3' -see below) in a detached, unheated, uninsulated garage with four windows. We are using the MHP approach with the 12x24" heating pad, and it's been working great.
Inside, the room temps been mid-upper 60's. Outdoors, the upcoming forecast is mid 70's daytime and mid 40's nighttime. We plan to use the MHP in the new brooder as well, and throw a tarp or something over the brooder before we go to bed to hold a bit of warmth in there. Hard to predict what the brooder temp will be with the heating pad as the only source of warmth. Mid 50's maybe? Obviously it will be warmer under the actual MHP where they sleep.
I'm thinking that for the next two nights, maybe we crack the window a bit to start getting them used to cooler temps before we take them out this weekend. Does that seem reasonable? Other suggestions for surviving the transition?