I have a different brand offset but the dimensions were the same as Horizons so I ordered the plate a few months ago. It works great, temps have leveled out all the way across the cooking chamber, and color of bark in my opinion has improved. The plate I ordered slid tight against the firebox end of the cooking chamber similar to the image of yours.
Hope you get everything sorted out.
Weber Genesis EP-310, offset BYC, Maverick ET-732, red Thermapen