As the owner I am biased of course but you might consider Frame Destination.
We have Mounting info here, and general framing info here (such as glass vs acrylic) as well as examples of how to put our frames together which demonstrate a simple mounting method:metal assembly & wood assembly

Even if you dont need or want to buy from us, the info should help get you started and we sell mat board with no window cut to any size you need.
Cheers,
Mark