Ok I guess I've got to teach all you guys how to do this.
There's obviously always enough room on the bottom sill to drill a 3/8 inch hole for a recessed switch close to ( but not butt up against) the outside edge of the inside window sill. Drill the hole straight down, not at an angle. Then use a counter sink to ream the edge of the hole so that even though the switch is sitting straight down in the angled sill the switch will sit flush and a little below the angle of the sill..... in a slight recess. You want to drill straight down so the drill bit doesn't come out of the wall before you get to the bay. Also, if it's new construction, and the molding isn't on yet, you'll leave the switch and or the wires exposed to the trim guy. Order your 3/8 contacts without magnets.
Now you take a dowel center punch and set it in the 3/8 hole and bring the sash down so that the dowel punch leaves a mark in the bottom of the sash, directly opposite the center of the hole. This indent will usually wind up somewhere between the weather strip and the inside edge of the sash.
Order 1/4 inch rare earth magnets which will fit between the weather strip and the inside edge of the sash. counter bore the hole ( ie flat bottom bit) on the bottom of the sash between the weather strip and the inside edge of the sash. Even if the indent from the dowel center punch didn't wind up between the weather strip and the inside of the sash, put the magnet there anyway, just lined up with the mark. The rare earth mag will still hold it in. I get Tane rare earth 1/4 inch magnets, only because they're convenient. They have to be cemented in and caulked, because they don't have a center hole.
On the top sash, order your surface contacts without magnets. Drill through the side top of the window as close to the top and sash, as you can. Above the track, where the hole can't be seen, if you can. Sometimes you can or have to drill up, but then you have to caulk/hide the hole. Use a surface mount switch mounted a half to one inch away from the side track. To avoid the magnet from being knocked off when the sashes pass one another, after you've mounted the switch, close the sash and put a mark on the sash where one end of the switch is. Just inside the switch side of the line, drill a counter bore hole for the 1/4 inch rare earth mag as close to the top edge of the sash as you can, without taking a chance that you wont leave enough meat ..... Cement and then cover the hole and magnet with caulking. People usually don't mind the surface mount on the top sash and I use it rather than another recessed, because it saves time.
Been doing it this way for about 4/5years now. It's a little elaborate with a number of steps but if you do it production style, ( perform the same operation on all the windows at the same time) it works great.
I haven't posted this before because I know Jack-off Bass is going to steal it ..... but what the hell.