Thanks for the suggestions everyone, one more question. Is there a specific pack that can be placed on top of the panniers?
There are commercially available top packs. About any tack store that carries packing equipment will carry them. Some you tie them down with a diamond or other hitch.Some have straps and buckles that you secure them with.
These folks carry a No Knot system.
https://www.outfitterssupply.com/horse-packing-equipment.asp