The best tester made is the Progressive INdustries HW-50C.. you plug in and if there is anything wrong.. that's as far as the bad power goes (the HW-50C) you use an adapter to plug into a 30 amp welder outlet and the wrong voltage is stopped at the HW-50C, you plug into an outlet with an open ground, Guess what,, The HW-50C tells you about it.
Great device.
What's mroe, some jerk taps a power pole and really strange things happen to the voltage for a few seconds (It leaves, comes back with a vengence, and then settles down.
The HW-50 C shuts you off for 3 mintues to give your air conditioners a chance to bleed down so they re-start normally install of stall, and the really strange stuff (Spikes) smooth out before you re-connect.
You want a cheap one.. I have yet to see one, But I would use the design you suggested
A big outlet box, 50 amp pigtail (male) to the box, pair of small (like 3ag, 10 amp) fuses to a pair of duplex recpitcals, a pair of 3 light testers (or move one back and forth) and a pair of volt meters.
NOTE: Fuse can be 15 or even 20 amp depending on the outlet but 1 amp will run the testers and meters or if you have room, put in a breaker.