The shop that made mine said that any good clear vinyl detailer is good and that you should put it on regularly. The reason it cracks is that the pasticisers dry out of the vinyl and it becomes brittle. You need to use a product for vinyl that contains palsticisers. It will soak into the vinyl and help replace what evaporated out.
I've been using Maguare's (or however you spell it) Clear Vinyl Deatailer. My "windows" are about three years old now and they look and feel like new.
I'm sure there are other good products on the market. Plexiglass cleaner is not the same thing I'm telling you about.
[EDIT: Do not use Rain-X or Armour-All or products like that as they will speed up the drying process of the palsticisers and cause the windos to fail prematurely]
I've seen people try to use toothpaste, pledge, and car wax, DO NOT use any of those. I've been in the boating bussiness for 25 years as a professional not a weekend warrior. Only use items like Plexas that are designed just for eisenglass, I've seen people swear by pledge but they windup replacing in 3 or 4 years because it makes the glass yellow and brittle.
Just my 2 cents for whatever it's worth.