TBT here: Some of the pictures were used previously, but to describe how to build one yourself, I'm using them again.
1. Get a small sturdy shallow box. I used a standard Amazon.com box. The dimensions are approximately 10" long x 7" wide x 3.5" deep. It could be smaller, but I think not less than 8x6x2.
3. After glue is dry, drill some access holes in the top. I used a "hole saw", a toothed round cutter that fits in a drill.
My holes are 1.75" diameter. I made 4 on the top and 2 on each long side. I centered my top holes 2.25" from all 4 sides. Cutting them by hand is possible, but difficult.
4. For the holes in the sides, make them almost flush with the bottom of the box. Then, cut straight down from the outside of those holes to the bottom of the box. Cut away the flap to make "mouseholes". Otherwise, they can't pull the treats out.
5. Now glue the bottom flaps together. You can apply weight through the top holes to get good contact. That can be a little tricky. I had some scrap wood that just fit inside the holes and placed a piece of board on top of them for weight. You might be able to use spring clamps more easily.
6. I painted mine. Spray-on enamel paint seems to work well. It takes several coats to cover the printing on the box.
And now, some pictures of Marley getting treats out of the box!