This balance board has been based on the Blackboard foottrainer. (example)

I wanted to make one myself and added brick studs for the balance bricks to be able to change the way the board works instead of using velcro and wood.

Like with the Blackboard, both boards are connected by wire. I used paracord and melted both ends to avoid slipping and because i didn't want to use a knot (which is another way to do it). You could tie a knot between both boards to create a distance.

Use more infill for the bricks and minimum 3 walls to avoid deformation of the roundness.

Also us a rubber placemat to avoid slipping of the boards.