How best to replace wood between weight and sash?

I have two windows that are missing the wood between the sash and lead weight on a rope behind the wall. Not in this picture but in another window in this place, that board is missing (in this 1920 year house), about a foot missing in each.

How best to fashion a board to replace that, and how to properly attach it so that it is flush to where it is supposed to be for the window, yet does not interfere with the lead weight on a rope behind it?

See picture: http://www.humanemontclair.org/extra/windows.htm

 

Typically it is a piece of 1X board with cut edges to sit flush in the opening. As it is not much more than cosmetic - you could easily replace it with a piece of 1/4 or 3/8 plywood. Simply cut it to size and attach it with a couple of wood screws. It does little more than hide the weights and provide a minor barrier to air flow.

 

You might need to remove your interior window stop (painted piece of trim on left side of pic) in order to expose the entire opening where this piece is missing.... but you also might be able to slip it in without removing it.

Normally those pieces are 3/4" thick, angled on the top and bottom, and just sit in place while being held with a screw. You might remove one of these pieces (by removing the screw) from one of your other windows and see if it fits on the other ones. If it does, there's your template. Just make a couple just like it.

Rip the wood to size on a table saw and bevel the ends (or rabbet them if that's the style of joinery used) to match the other piece.

 

As usual, Xsleeper has the "right and proper" way to fix the problem and I bow to his answer - assuming you've the tools and ability to follow his easy instructions. Mine is the quick n ez method for a guy with a handsaw, hammer, and not much else.

 

I was just trying to give a longer answer. Either way will work, like you said, those covers are pretty much just cosmetic.

 

Thanks to everyone that responded!