If you're not using FVC walls there may be some modifications required. My walls are thin as I did not do any furring strips. Instead my walls are 1/2" bamboo panels that go right up again the metal and held in place with plusnuts. Due to this I had to make 1/2 spacers to increase the overall depth of the wall around the window. These spacers fit between the bamboo panel and the outer trim ring from FVC. It doesn't look bad so I'm happy with it and doing it this way was easier than actually making my own trim rings.
The actual inner trim rings from FVC do not fit exactly around the windows I purchased (CRL VW1033L and AW1033) they basically do not make full contact to the actual van wall. Luckily I planned to use butyl tape as a means to minimize rattling from metal on metal contact. Once everything thing was put together you don't really notice what I did - despite me not being an expert woodworker.
Product improvement suggestions would be:
1. Offer an aluminum or non-metal option. (With places to put in concealed magnets for window coverings).
2. Provide an easier method to trim the rings for shallower walls. (The inner ring threaded nut is to far out, maybe a second one so when trimming it you have another spot to screw into)
Final rating is I would suggest using these trim rings because I value my time more than trying to figure out how to make these by hand and they look good.