Instead of mounting multiple whiteboards side by side and having a large unusable gap between from the aluminium frame, you can carefully remove one side of the frame from each whiteboard (or two, if it's not on the end of the row) and use these parts to attach them together, resulting in a much smaller unusable gap. The connector takes the place of the plastic corner pieces where two boards meet, and the plates can be used to add a bit of extra strength/stability to the joint (as many as you like, though the boards we used had four screws holding the side of the frame on, so we reused those screws for four plates per joint) - just lay a plate across the joint and screw through the holes into each board.
The whiteboards we used were Penrite branded, though others may also work if they use sufficiently similar aluminium edging. You may also find that the frame is glued as well as screwed on to the board, in which case the frame may end up getting bent during the removal process (and you may end up with small chunks of the fibre board being torn out), though older versions of the same whiteboard seem to use a much denser backing board and no glue.
100% infill used as these are load-bearing parts. Supports touching print bed only, to support the "wings" on the connector.
Designed using OpenSCAD, using the original corner piece as a reference. The initial revision tried to stay too close to the design of the original corner piece, resulting in insufficient strength (especially when coupled with a poor quality print), so I redesigned it to increase the strength without extending beyond the whiteboard surface (and cleaned/calibrated my printer). I haven't yet designed a cap/cover (equivalent to that found on the original corner pieces), but there are mounting holes for one.