Lundy,

The outside diameter of the HW inserts is ca. 14.6 mm.

The standard HW element pack consists of three (3) posts (fat post, middle post, and pin head), and two (2) aperture - please see below.

I have tried the metal FWB / Anschutz aperture elements (ca. 16mm), and these do not fit the HW tunnel.

Some of the old Diana elements (from 35/50) might fit, but again they were well supplied with posts not apertures

That is why I went over to using modern foresight tunnels.

Some chappie could make you up a suitable plastic sight element ? ( they are only a disc of plastic with a hole drilled in the centre )

Have fun & a good Sunday,

Best regards

Russ