My 189FBR came with an extra skylight cover in the box. Icon brand p/n 01818, Skylight, SL1414C, Clear. Description says flange size is 18 x 18, and dome size is 14 x 14 x 4. Clear polycarbonate.
I have not replaced the original because, well, let sleeping dogs lie, and the sealer over the edges, and screws is intact.
Not offering to sell it, just saying they are available.