Hi,
I have a recent thread with same problem. Having a look on the usual auction site I came across this:
http://www./itm/Merit-Optical-Attach...item3f4c28fc17

Probably been looked at before (was that nearly a pun)? Looks like an expensive sophisticated adjustable insulating tape competitor.
Wonder if I could knock 1 up from an old camera lens?

New pistol arriving shortly, see how I get on with 1.5 readers.

Rgds, Rob