Mark

What’s the diameter and thread pitch of the filter wheel you have? If from ZWO, I suspect metric, you could use one of the spacing rings from Baader such as this for M68: https://www.baader-planetarium.com/en/accessories/adapters-imaging-accessories/m68-zeiss/baader-m68-varilock-15-20mm-extension.html
Baader also have these for M54 and M42 I believe. And they also sell AP to metric adapters for your flattener. 

Precise parts will certainly make you a very good part but the Baader route is also available and likely cheaper, especially if you are not in the US (I don’t know where you are). 

Or you could machine an adapter if you have access to a lathe?

Roberto