Narrowband Crescent Nebula revealing the OIII

[rubecula]

My first attempt at processing Ha and OIII subs of NGC 6888 was really garish but at least the OIII was apparent.  I'm trying to take a more gentle approach but no matter how I try I can't reveal the OIII without spoiling the rest of the image.  In Photoshop I've tried Selective Colour, Select by Colour range, Curves on the blue channel and none of them work satisfactorily.  Here's where I'm up to now:

The OIII can be seen very feintly so I know it's there (as it was in the garish version).  Is this a job for masks?  I've not really used masks and when I've tried it's obvious what I have done.

I'd be grateful if someone could give me some pointers as to how to tease out the OIII without impacting the whole image.

Thanks