I have wondered about obtaining or making a perforated cover for the amp, similar to those on the Radfords. Also noticed an old advertisement on here recently with a cage for sale for an Almarro 318b: Which is a similar thing.

Having tried a few Googles I drew a blank so thought it worth asking if anyone has bought or made anything for the ST20 ?

If something fell onto it It'd be costly & time consuming to repair. Although it has now been removed there used to be a large wall clock above the amp - and had that ever come off it's fixings, it would have wrecked the Leak.
I've noticed a lot of 'architectural' uses of perforated metal decorative panels in the last few years, which suggested there may be more companies turning to the production of such items. I even noticed the subtle use of large powder coated panels at Meadowhall (Sheffield) with perforated details that reflect the City's historic manufacturing of cutlery - the cut-outs are shaped like forks !
If somebody can go to that trouble, it shouldn't be too difficult to make a box for an amp with Radford type perforations.