This 1x1 Gridfinity Holder will hold up to 3 Breadboards. Just print at least 2 Holders and space them to fit your Breadboards length. The regular sized Breadboards like BB830T from BusBoard Prototype Systems (or a cheaper clone) will fit perfectly with a 2x1 Gap between the two Holders.

Note that I redesigned the base of the holder to save some material. you should be able to print that without support even if your printer does not handle bridges too well. Its in the end just on the underside of an organizer :)