I haven't seen the program in a few years. Candy was the one who would tune in and my watching was mostly when passing by or sitting down for a bit of rest or relaxation.
So just for the heck of it Dr. Google was called to task > holmes on homes electrical panel < was the search term. Screen shots were taken of three panels. The first was posted in 2022, the second was posted in 2019 and the third in 2020:
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Apparently it doesn't matter whether the panel is mounted in a vertical or horizontal orientation.
Notice the third one doesn't have a cover. If it is inside a building there likely isn't a likelihood of it getting rained or showed upon. Also note the labels are oriented as if to be used in a vertical orientation.
Another thought, if the box is mounted horizontal outside wouldn't it make sense to have the door open upward so if it wasn't fully latched it would still help to keep out the weather from gravity holding it down?
jtk