I got fed up with my breadboards curling up when I joined two together to hold larger modules, so I whipped up a quick base to stick them into and hold them straight.

Print the base in PLA or PETG, and optionally print the foot in TPU and use glue or double sided tape to attach it to the base to help stop the base sliding on your work surface.

I've published the CAD in Onshape, and set the dimensions of the board in a couple of variables so you can easily adjust and re-export if required.