WARING: I have not actually used the KOP chassis for actual competitions, and have limited experience with it in general.
So generally a battery strap seems to work just fine. Essentially just bolt some polycarb into the robot as a base, and have some slots cut out for the strap. I would recomend other things to be placed around the place the battery actually sits to constrain it’s movements further.
Battery would go where the transmitter currently rests. This is subotimal positioning for actual robots, and not the lack of constraining blocks. The constraining blocks would be relatively easy to mount, just use wood or spare metal and bolt through it with some washers, sort of like this:
Agreed, 1" aluminum angle works well here too (same concept as bot with purple noodle bumpers) if you need to create a little more space for something else.
Going back to OP, the current AM14 outside plates have essentially the same construction as the horizontal battery holder. According to the drawings, the flat plane is 0.625" from the center of the hole. Putting two of these in holes 8.5" apart in the end plate will leave you 7.25" gap. The batteries are nominally 7.1", so you’ll likely want a bit of padding in there. The other pieces of the horizontal holder are a flat bar and a way to secure the bungee; shouldn’t be difficult.
Thanks for everyones input! If anyone else needs a horizontal battery mount, I recommend using aluminum angle as Skyehawk mentioned. Keep it simple and make sure the battery is secure and easy to swap.