E-bay windows , no trim ring .... cut my 3/4" door/side as neatly as possible will insert as per Mad Jack ...inside should be a small seam where the window meets the ply(URGH!!!) ...gotta be some small plastic trim that would cover this seam and still look "finished" ... I'm looking around too , I think a Marine supplier may be able to supply trim ... or how about just caulk , fill and paint it ... painted trim being very traditional ...and handy ...
Perhaps CURTAINS ??????Cover em' up...
or , how about a thin cardboard /headliner material ,/Masonite 1/8"/ whatever flat trim ring covered with the curtain material , or leather,ya' Mamas' old shower curtain / whatever looks best ... couple trim screws with finishing washers... like in cars... we need to visit a local Upholstery shop and get some ideas ....