Cut the plywood into size.    {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/5\/53\/Make-a-Bicycle-Saddle-Bag-Step-19Bullet1.jpg\/v4-459px-Make-a-Bicycle-Saddle-Bag-Step-19Bullet1.jpg","bigUrl":"\/images\/thumb\/5\/53\/Make-a-Bicycle-Saddle-Bag-Step-19Bullet1.jpg\/aid4393664-v4-728px-Make-a-Bicycle-Saddle-Bag-Step-19Bullet1.jpg","smallWidth":460,"smallHeight":306,"bigWidth":"728","bigHeight":"485","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a> (screenshot)<br>\n<\/p><\/div>"}   Visualize where to mount the bags. Mark the areas where you will put holes on the bag where the U-bolts will go through.    {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/2\/2a\/Make-a-Bicycle-Saddle-Bag-Step-22Bullet1.jpg\/v4-459px-Make-a-Bicycle-Saddle-Bag-Step-22Bullet1.jpg","bigUrl":"\/images\/thumb\/2\/2a\/Make-a-Bicycle-Saddle-Bag-Step-22Bullet1.jpg\/aid4393664-v4-728px-Make-a-Bicycle-Saddle-Bag-Step-22Bullet1.jpg","smallWidth":460,"smallHeight":306,"bigWidth":"728","bigHeight":"485","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a> (screenshot)<br>\n<\/p><\/div>"}

Summary:
Visit your nearest thrift shop and buy two identical bags. Measure the length and width of both bags. Transfer these measurements to a piece of plywood. Insert the plywood into the bag. Install a bike rear rack (this is where you will mount the saddlebags)  Place the bag on the side of the bike rear rack. Poke holes on your markings. Get your drill and make holes passing through the plywood inside the bag. Place the U-bolt on the the bike rear rack. Mount the bag and secure them with the U-bolts and nuts. Do the same steps for the other bag.