NICE SHAPE! They are going to be SWEET!!!!!!
I use heavy card stock from the craft store like poster board, just because it is easier to trace and the templates last longer.
Plexi would work in between the sheets of leather. But I personally use metal. Less prone to breakage/melting. A sheet of aluminum because of the weight factor.
Less weight the better!
Formica would also work, but aluminum is much better, also since the bags are going over the pipes and might over time droop and start touching the pipes, especially overloaded )and they do get overloaded on trips).
ANY PLASTIC MATERIAL WILL START MELTING. Metal will be much better!
Aluminum is normally about 3 bucks a square foot so much cheaper than plexi or formica. And it is what you need , they will cut it for you if you give them the dimensions.
I have fixed A LOT of bags, and they are always lined with metal, because bags that size always burn/rot through the leather over time. Not your fault, but the fault of the bike owner for not cleaning and taking care of their bags like they should...
Take the aluminum and measure the distance for back bottom and front and cut it into a rectangle, put it on you bench and make the marks and fold it into a U shape so it is structurally strong. Drill out a few holes so you can rivet it to the front bottom and back sides to help add internal support. That will help them from drooping over time.
That is my 2cents:)