>stove element protectors (pretty round things with pictures on),

Old fashioned cake or biscuit tins may also suit this role. The main part of 
the tin may need trimming down to a suitable depth, but for a one-off could 
be ideal. May also be able to get rectangular versions that might suit some 
form factors better.

Another "ready made" one to remember is the zinc alloy boxes made in the UK. 
These come in similar sizes to many of the smaller plastic boxes, and are 
very easily machined. 

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