I know this has been mentioned by folks in other threads but I am here to bare witness. I've tried just about all of the suggestions I've seen posted here on how to "easily" clean the inside of the MES window. Most work good, some better than others. Unfortunately, being in South Florida, I don't have a fireplace or wood stove to use Bear's wood ash method which sounds the cheapest.
What I have found that works the BEST (for me at least) without ANY chemicals is a bucket of hot/warm water and the Mr. Clean Magic Eraser. I just did the window on my MES 40 which has been sitting since a Mother's Day smoke and you couldn't see through it at all. The crud was baked on and holding tighter than a tick to a hound (or a Wall Street banker to a dollar, whichever you prefer). Broke out the Magic Eraser and in just a few minutes it was like new. I wanted to try this nifty sponge as I just don't care for the idea of using any chemicals on the inside of my box. I'm sure glad I did. They are fairly inexpensive at Wally World for a 2 pack (won't get more than one use out of one). Give it a try!!