A RHP is mounted on the ceiling of the cage.
If you are going to add a shelf you have to remember that this will then cut the cage in half so there will be a portion of the shelf that is only six inches from the shelf. Also depending on which RHP you go with may make it a bit harder to clean the shelf. I have three Pro Product RHPs that are very thin but then the one in my PVC Cages cage is a couple inches thick. In the cage it is in it doesn't matter but if it was in a 12 inch cage with a shelf only 6 inches under it that would only give you a four inch gap between the shelf and the bottom of the RHP.
If you think you want a RHP you may want to get a few more inches in height.
If you go through this site as if you are picking out a page it will show you pics. In some of them you can see the RHPs. Some are black and others are white. Select different size cages cause it will show you different pics. If you click on "Accessories" and then select "perches " it has a good pic showing the RHP mounted to the ceiling.
http://pvccages.com/