No great suggestions for cleaning sadly but
we bought interior window inserts from here
Our house also has exterior storms and is in the historic district so any other type of replacement windows would be very expensive. These have an edging of foam around the outside which is impressive at blocking drafts and they just pressure fit into the window. I was worried about measuring accurately at first but customer service was very helpful on the phone. Each insert is custom made to fit your window and prices vary depending on size. I was initially nervous about buying so just did a small order then was so pleased ordered for all the other windows. We store them in the attic during the warmer months and put them up when the chill begins.