I'm going to assume that you want to mash 1.75lbs of flaked barley, and steep the 17oz of roasted barley. From this assumption I'm going to figure that you'll want to mash 1.75lbs of the pale malt with the flaked barley. So I'll walk through how to calculate how much Liquid or Dry Malt Extract...
The key is really specific gravity. Specific gravity is a measure of the density of a liquid, and an indicator of the amount of sugars dissolved in wort. In "Designing Great Beers", Ray Daniels introduces Gravity Units (GU) as a way of calculating how much of a particular sugar source you need...
For temperature stability, I'd put the fermenter in a tub with water slightly higher than the level of the beer. I'd keep a frozen water jug or two in the water to keep the water temperature close to 32°F. Don't forget to change out the jugs as they thaw, usually once every day or two depending...
I've not cut the hump in a chest freezer, but I think you might be able to use sheet Styrofoam from the local big box home improvement store to insulate the resulting hole. That stuff isn't too expensive.
Good Luck