The Contented Little Baby Book (which I found utterly mental, BTW - as a C-section mum the very idea of scheduling everything to the minute from the moment of birth was simply) did have some wisdom in it. The author advocated using a pelmet, not only at the top but at the sides too, so the curtains were recessed into boxing. I no longer have the book but presumably painting the inside of the boxing matt black would reduce reflections too.
I made quick-and-dirty curtains for the room at MiL's where DS slept when he was tiny, simply by getting lengths of blackout (the version that is undyed/off white on the room side/fabric side and has a whitish rubbery coating on the window side), stitching lining header tape (ie, single width that sandwiches the top of the fabric) and not bothering with a hem, which would have added needle holes in any case. By making them full enough there wasn't enough light to cause a problem, but that room did have a pelmet.