I personally like "Graphic 23" backs, which are, I believe, earlier iterations (with knob wind) of what became the Singer back. I don't have any to fit 6x9, I do have 2 examples, one for 1/4-plate and one for 4x5", and find them robust and good to use. I think you can certainly find one for much less than an RB67 magazine.

Here's one if you're quick: