I used to do some crafting with acrylic sheets back in the 80's. I would use glue specifically formulated for acrylic. It doesn't glue the acrylic, it actually melts it and then fuses it together. If you have access to a granite counter, I would lay several sheets of cooking parchment down, lay the ruler on top, follow the adhesive instructions (it will get drawn into the crack by capillary action) and add a few more layers of parchment & a bit of weight. The granite counter will keep the ruler perfectly flat. The bond will form pretty quickly, but you want to wait a couple days before using it.