The frames are metal and had no clips or retainers installed - the screens were not in the window frames when I got the trailer.
I laid the screens flat on a workbench and duct taped the four sides of the frame to the bench before I removed the old spline and screen, so I'm confident that the new screen was not too tight as to deform the frames.
Are the plastic clips referred to a spring type clip that actually retains the screen in the window frame? Local RV dealers/service shops have no clue..
By the way... thanks for the responses!