I hated cooling or heating the outside (due to the loss of air to the outside through the sliding glass). I tried a rubber, sticky backed, weather strip I found at Lowes. It worked reasonably well. It took a little doing to get it between the glass (without the sticky side grabbing before I had it in position. [I'm wondering if cutting a piece to length, putting it into a sandwich bag <adhesive side still with the paper backing in place> and putting it into the freezer for 5 minutes would lessen the adhesion ability of the sticky side long enough to make working it between the 2 pieces of glass would help... ?) I used a narrow, thin metal bladed spatula to poke it gently into position. Then worked the blade between the "rub" side and the other window, pressing it into position and working it down the window to press it into place. It works faily well (not perfect...

). I think a manmade material (less chance of mold buildup) felt strip would work better. Have been on the lookout for that but as yet haven't found it.
Bill