Teardrops n Tiny Travel Trailers

So I purchased these windows from an Ebay seller, and they did not come with a trim ring. They do not appear to be set up to use one either. Does anyone have any installation insights into the recommended method for them? The rail/channel where I would expect an inner ring to "suck" the window in actually has the screen sliding inside it. I have plenty of room to drill the corners and attach with stainless bolts/neoprene washers in that manner, and likely at 12, 3, 6 and 9 o'clock. I am also planning to use a layer of butyl tape behind it.

more pics, these are all on the inside of the window.