When I did the pg500 I did the vertical intentionally because at the time I was fixated on making somewhat traditional style pastor tacos and gyros. It was most likely also the easiest solution. It really did work pretty well, but I could see a really well designed horizontal version being more functional.
I did a rotisserie build on my louisiana as well. Something you could do for one less hole being drilled is a high powered magnet. I bought this one at harbour freight. I was warned that the magnet may not hold up to the heat, but I haven't moved it since I did it well over a year ago and its held up fine.
I got stainless disc magnets from harbor freight as well to cover up the hole that I drilled in the top of the pg500. If you went through the chimney with the hole, you could use those to cover when not in use. Also, unless you wanted one, a bracket wouldn't be a necessity to hold the motor. I use two handled magnets as in the photo to hold the motor bracket onto the metal.
So, in total, you could probably get away with just two drilled holes. One in each side of the chimney.