Modifying electric water heater for easy temperature control

Making it easy to adjust is defeating the purpose of the cover. But since this is what you want, here is a way to do is make it easier to adjust without removing the cover.

1. Replace the adjustment screw or bolt with one long enough to stick out of the cover approximately 1/2 in. The screw head type could be hex, flat blade or Phillip head. A hex nut could be used on the lower thread to clamp down the adjustment knob with the upper part extending out past the cover.
2. Where the screw would protrude through the cover, drill out a 1/2" hole in the cover so the screw can protrude through.
   

That said, It is strange that you are trying to set an exact/precise temperature. I would just mark the two points that you want to set for winter and summer. The installed thermostat is not designed for setting to a precise temperature.

That's not a thing you want to do. You need to keep the heater temperature around 140F in order to stop breeding bacteria, including legionella. See the discussion here.

This legionella issue is relatively new and not widely known yet, but it is legitimate.