Appeal for help

I've been struggling to find suitable replacements for the four 15 inch diameter, circular, 'spectacle' cab windows for Sentinel 7109's restoration.

Front pair of 'spectacles' from an old photo

Ideally, I would like genuine Sentinel windows as used on many of their early locomotives but alternatives may be suitable. They pivot about a vertical centre line as shown in the right of the picture.

The original windows were lost during transfers between previous owners and have never been found.

Can anybody help please?

My Email address can be found as the Steam Locomotive Officer here: http://www.sentinelwaggons.co.uk/7.Contact/Club%20contacts.pdf