Have you tried covering the open side of the tin with two layers of
chicken mesh slightly offset.

Dave

Thanks for the reply. I did briefly try that, but it wasn't very successful.
I think part of the problem is that the biscuit tin appears to have a layer
of laquer on it, and the chicken wire has a slightly oxidized Zinc coating,
so it may be difficult to get a good electrical connection between the two,
which I understand is important. I think I will try this again.

What you need to do is use plenty of nuts and bolts with large washers to
hold the mesh in place and yes, scrape the laquer off. Blimey this is
taking me back a few years to Amateur Radio days.
RF will leak around holes so good firm electrical contact is essential.

Dave