Normally fridge magnets are those brown flexible plates used in a halbacharray configuration.
These are so weak that they have to be used in these configurations to make them stick.
Do not use those.
Instead order some neodym magnets from ali for example. Those come in different sizes. I would buy those that fit in these tubes on the bottom of a 2x4 brick. You can glue them in there with some epoxy. Make it so that the magnet is inset into the tube by half a millimeter. If it sticks out you get scratches on your fridge. Wifes hate this.
I once built a wooden marblerun on my dishwasher. 3 cylindrical magnets of 3mm in diameter and 8mm in length were sufficient to hold the tracks. Even when the balls were bouncing on them.
For a model I suggest flat models built from plates or textareas, maybe a greeting or a TODO-list. If you want larger models, just increase the size or the number of magnets. (and/or the size or number of the fridges.)
There are magnets that can lift your fridge, so that should be enough to hold your bricks.
If you have Kids, just throw the magnetized bricks into the kids room and count to 10. Then you see what is possible with those.
Warning: Magnets are dangerous! You should not use magnets when you have small kids which could swallow those magnets.
Even if you glue the magnets into the bricks they can become loose!
