Well it didn't used to be a legal requirement in this country - is it actually a legal requirement ? If it is, then there must have been an assessment done. Given the normal opinion that electricity and water should never mix - it seems odd to insist on putting stuff outside, given how flimsy the cabinets are.
If I'm ever forced to do that, I;d be looking for a galvanised steel box, with a mesh cover on the door so it can be rendered and painted to match the house. What do you mean, it'll block radio signals ? Why would I want my meter talking to the world every half hour !
I'm still trying to think what to do with the fugly box a previous owner stuck on the front of this house for the gas meter. Of course, not being built in, it's even more obtrusive than the built on ones.