I agree with this. I had an Alfa Spider once that had the roof slit to remove the radio. I dare say the roof cost way more than the radio. That translated to me never locking any soft top, Morgans included. Here in the US, nobody wants to steal the cars anyway and most radios are out of sight.
Cheers,

Mark

1934 SS MX4
1938 Series 1 DHC
1951 Plus 4
2005 Roadster