After a a bit of searching online I came across Slay My Print, who had just exactly the frames I was looking for.
Ordering was easy. Frames arrived quickly and very securely packaged. They are just perfect - plain black solid frames, well made and just what I was looking for. The plexiglass had a protective film on each side which was easy to remove, leaving a perfect scratch-free finish.
I would highly recommend these frames and would purchase again. 10/10