The glued perspex box is made up of a lid with four sides all of which have to be glued together with a special clear glue. This can show up small bubbles it the joint but are usually small and not a distraction. This method of making perspex box frames while beautifully sharp, has a disadvantage it that large frames can be a little weak and must be handled with care not to damage them. But you would handle art with care anyway!
Another aspect of gluing the sides is that they can be made of another material like an opaque acrylic which can give an interesting look. The base of these frames can be made from either a very rigid foamex or a wooden sub frame with a foamcor on top, both can be seen through the side wall of the frame and so are usually painted white.