I would strongly suggest you just spring for the whole house fan. I installed one several years ago and love it. Its just a basic model off Amazon. I bet a used one off craigslist would be fine as well. They are extremely simple and if they look to be in decent shape and they run they are probably good to go.
Rather than install the fan in the main house, I installed mine in the garage then added a screen door between the garage and the house. This solves the issue of insulating the WHF in the winter/letting heat in from the attic when the fan isnt running. I just open my windows, open the door to the garage and run the fan. The fan came with a timer that can run for various lengths of time. I have thought about connecting it to something more intelligent so it can run when it is coolest. Maybe for an hour or two before I get out of bed. It would have to somehow sense the pressure drop from inside the house to the attic to ensure windows are open.
The install was really simple. Ours mounted on top of the rafters then we boxed in around it to make a tunnel of sorts up to the fan. No rafter cutting required. The instructions came with the fan and I think its a pretty common technique. The louvers mounted to the bottom and miss the rafters when they open and close. I added a dedicated circuit to the breaker as well.
Definitely install the fan in the early early morning or late at night. You do not want to be working in a hot attic. They are terrible. I started too late in the day and was dying towards the end of the install. A couple more advantages of the garage install: 1) The garage isnt insulated so no insulation to move out of the way. 2) the breaker box is in the garage which made connections easy 3) my attic access is located in the garage so I didnt have to carry the fan very far to install it 4) way easier to clean up a mess in the garage than in the house.
Do look into your local codes. There is some concern about a WHF in a garage being a sort of fire ladder/carrying flammable fumes into the rest of the house. For me the advantages outweighed the risks. Also if you live in CO and want some help shoot me a PM.
