These are the measurements that I used (and in millimeters). Of course you use the measurements that is suitable for you and the animal you are going to keep in the cage.
I only mention the outer measurements - since the inner measurements depend of the thickness of the material you use.
The "inner height" is 50 millimeters lower than the outside height. I raised the bottom to fit a heating mat beneath the cage. This is not necessary to do, but since I live in Sweden and it can get quite cold during winter I need a heating mat.
I used to build in Plywood. I have no idea if this material is available everywhere, but I thought it was an easy material to work with and it is fairly cheap.
Be sure to only use non-toxic material. When you buy something - ASK if it is non-toxic. It is better to ask one time to many than to have your animals suffering later on.