I made mine out of popsicle sticks and just cut off the rounded ends. Started off with 3 x 3 but thought they were a little small so now my standard is 3 x 3 1/2. Have built several larger ones 3 x 4 and even 3 x 5. Just use some square basswood for the rails between top and bottom. Cheap and easy to make.
I've even made some very large custom ones, one of my flatbeds load is actual full size bricks. At .62 each a bargain
