Ok, the dog weighs about 7.5kg and the space required is about 30cm x 60cm, 20cm tall. With several materials available, I design a concept based on that data, building the design without being able to find the material would be really difficult.
Avoiding sharp corners, I quickly changed the design a bit.

This step requires welding, grinding, drilling and cutting skills. I used a V3 iron with a thickness of 2mm for this design. Remember to use full protection when working.

The wooden board helps to bear the dog's load better, I designed 2 horizontal and slope modes but the latter completed the working horizontal mode is good enough.

The protective metal mesh helps the dog focus on running without falling out..

Assemble the rubber mat, 12VDC electric motor and spay paint to complete. Test operation by running the machine continuously for 2 hours. And finally here is the result after running the test.