Sorry to hear about them being out of stock, I got lucky when I ordered mine I guess as it arrived in a few days.
For an alternative have you looked at the Artesky 2" Filter Adapter for Celestron 8" RASA? You can screw in 2'' filters, it basically replaces the stock M42 adapter, while being thicker which helps getting to the proper backfocus. It claims to change the needed backfocus to 17.5mm, which is exactly what your 1600 should have with the stock 11mm T2 spacer on (11mm spacer + 6.5mm camera window to sensor), so you just screw it on. It also comes with a 0.5mm aluminum spacer. I'm a little confused on whether that's needed or not. Measuring the thickness of the adapter it's 11mm, so that would be 11 + 11 + 6.5 = 28.5mm, and the RASA 8 manual states 29mm is needed, so that would mean it is required, but it's odd as they don't mention it being needed. Of course, there's a number of other factors like whether you kept in the clear window, or the thickness of your filters, etc... There's a whole thread here on finding proper backfocus with the RASA 8, so expect to play around with it, but it should get you close.
I also must apologize that I ordered the last one off agenaastro (just received it today), however it looks to be in stock on teleskop-express -- that would ship from Germany. I got it to possibly shoot OSC during galaxy season as I'm not sure I want to switch out 4 filters, or maybe shoot OSC one night and Mono the other for luminance. In both cases I wouldn't need to switch my astronomik L-2 filter. Also wondering if the Artesky adapter will improve any tilt/backfocus issues since it's just one solid piece.
Finally, if you're feeling particularly adventurous and want something right now, here is a DIY video by Roswell Astronomy on replacing the clear window in the rasa with your own. I'd probably only do that if I was only shooting OSC, or if I wasn't going to use the clear window anyway, but it is possible.