more newbie questions - buying used radius corner windows

OK, so I'm finding a good number of used radius corner windows on Ebay that say "no trim ring". For example:
http://www.ebay.com/itm/RV-BLACK-TINTED-UP-DOWN-WINDOW-R-O-15-X-15-USED/350943827742?_trksid=p2046732.m2060&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D3148525857801934086%26pid%3D100040%26prg%3D1011%26rk%3D1%26rkt%3D4%26sd%3D350943827742%26
So my understanding is the trim ring is just a decorative piece? Or is that the clamping ring? If it's missing, could I fabricate my own trim ring/clamping ring out of, say, plywood? Trying to understand how I might use one of these things, they sure look much nicer than square windows IMHO.
Please forgive me if this is well covered in an existing thread. I really am trying to search for stuff before starting a new thread.
http://www.ebay.com/itm/RV-BLACK-TINTED-UP-DOWN-WINDOW-R-O-15-X-15-USED/350943827742?_trksid=p2046732.m2060&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D27%26meid%3D3148525857801934086%26pid%3D100040%26prg%3D1011%26rk%3D1%26rkt%3D4%26sd%3D350943827742%26
So my understanding is the trim ring is just a decorative piece? Or is that the clamping ring? If it's missing, could I fabricate my own trim ring/clamping ring out of, say, plywood? Trying to understand how I might use one of these things, they sure look much nicer than square windows IMHO.
Please forgive me if this is well covered in an existing thread. I really am trying to search for stuff before starting a new thread.