Hello Julie,
A5 (5.8 x 8.3 in) is rather large and pouring urethane epoxy flat on wax paper will work but what you use for a form and the end result will be tricky. For the most part silicone makes the best molds.
Even if casting ~1/8″ thick the end result will likely be “bendy” and will likely also crack. Cured epoxy is like concrete; it is super strong in compression but weak in torsional “bendy” forces.
Check out styrene model building parts used for architectural models. Cut yourself a frame and pour into the frame. I’d also use triangular gussets to make the frame resist stresses. They can be cut in any decorative manner you choose but you want something to provide structural strength and resistance. You can even build some “book cover” hinges from styrene. It is easy to work with and finish.