Quarter tone flat accidental?

In a new piece I’m writing, I’m attempting to use a quarter-tone flat accidental: the natural sign with an arrow pointing down. But Dorico seems to allow only the flat sign with an arrow pointing up, instead.

Last December I wrote a piece for trombone and electronics (with the previous version of Dorico) in which the altered natural sign works. If I open that file, the altered natural sign is still there; but in the new piece I can’t get it to work. Attached is a jpeg, as well as a single-bar Dorico file for solo double bass. If anyone has ideas about this, I’d be very grateful - thanks!
quartertoneflat.dorico.zip (428 KB)

In write mode, click on the key signature tool on the right (2 sharps). Scroll to the top of the panel that pops out, and choose “Equal temperament (24-EDO)” in the drop-down. Scroll back down and all the fancy accidentals will be available to use. Screen shot below, the mouse is hovering over the dropdown you need to click to add this

Thanks, I have all of that set up already, as well as the atonal key signature. What’s not working is the enharmonically equivalent notation. For a quarter tone below C, I would like to use a natural sign with arrow pointing down. But whenever I click that button, instead I get the flat sign with an arrow pointing up. So it’s the “same” pitch, but not with the desired enharmonic notation.

Has anyone else encountered this problem?

ah I see. I wonder if the Respell option in the Write menu works for this problem?

If you definitely don’t want to use the accidental with the arrow pointing up, you should be able to delete it from the Edit Tonality System dialog, which will eliminate the option with the equivalent accidental.

Thanks Daniel - the Edit Tuning System dialog worked, once I switched to “Extended Helmholtz-Ellis accidentals (just intonation)”. And thanks CelloG for the suggestion - actually, the Respell option didn’t work.

In the attached screenshot, you can see that now I have two altered natural signs appearing in the right-hand panel; the second one, which used to the be flat sign with an upward arrow, works; but the first one, the natural sign with down arrow, appears not to work no matter what. Sometimes it would be nice to be able to use both glyphs. Am I doing something wrong? Maybe it’s a bug?