Thanks for the advice. My plan is pretty much stevehull is suggesting with, oddly enough, matching the 1/4" gap with 4 layers. I wasn't sure if going with 3 layers with 1/3" gap would be any better but it seems that if my frame isn't strong enough that it might make more sense. I'm also not exactly certain how I will build this with 4 layers. It seems to me that 2 layers is easy by building a frame and putting a layer of plastic on each side of the frame and putting it in place. I might try making two of these, total of 4 heat shrunk layers and spacing them appropriately to make 4 1/4" gaps(1st gap between a layer of plastic and the existing window with another layer between each plastic layer). If I include the double pane window that would be a total of 5 gaps.
I'm going to have to do some research on what woods are the least flexible and available in the appropriate sizes when shopping for lumber. Aesthetics as far as the wood goes aren't too important for me so I'm not picky on the color. The existing trim is oak. Thinking about it, I might just look through the millwork aisles for pieces that I could use since most of that is suitable for nailing and is the appropriate thickness for decent strength and looks good.
We'll see once I get a chance to take a look, maybe this weekend, hopefully I won't be as busy as last weekend.
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