Buy a two dollar and ninety nine cent Styrofoam ice cooler.
Buy a small box of Mortite. (very cheap)
Duct tape, small roll.
Determine how much each side of the extensions can be drawn out.
Measure those distances.
Using a sharp knife, carefully cut as many panels of Styrofoam as you can out of the cooler and its top to match the measurements of the panels.
(What you are looking to make are nice, thick pieces of insulation.
I can usually get one from each of the narrow sides, two or more from wider sides and bottom and a couple more from the top.)
Duct tape as many panels as you can onto the extensions, inside and out.
The thicker the add-on panels the better the insulation.
(and remember you are going to have to get the damned thing back in the window, so leave yourself a little wiggle room.)
Put the air conditioner back in the window.
Use strips of the Mortite to fill in the cracks between the extension panels and the sides of the window. It's clay on a string. Just press it into the gaps. Use more to seal between the bottom of the window and the top of the A/C.
(Hopefully you put a nice fat piece of foam under
the A/C before you put it back in the window.
And another in the area between the two window frames.)
If it's too difficult to insulate both the inside and the outside of the extensions, just do the insides after the A/C is in the window and you've sealed up all the cracks and leaks.
Joe(Send me Fifty bucks --half of what you will save on your a/c bill this summer.)Nation