Another alternative and i know this is probably pretty unlikely, is finding another R5 owner who doesn't care for having a window(but has one) and will do a straight swap with you. £31 inclusive of delivery isn't too bad though, probably about £10 more(give or a take a few quid) then you'd pay buying them at original pricing with delivery on top. You will need to speak to Fractal and check if they expect to stock it again, else you might be sat waiting indefinitely for stock that's never coming, as it's technically an end-of-life item now.
In my experience, whilst responses aren't instant, they do get back to you with an answer.