Hello there! I am not sure what your porch floor is made from but I am sure that building a raised garden over anything but dirt may not be a good idea. Your raised garden will contain lots of dirt, will be heavy, and will require good drainage. If you plan on building a raised garden ON a deck like floor or a cement floor it may ruin what is underneath the garden. Perhaps you mean that you want to build a raised garden in front of your porch on the ground?
We've made several plant stands that are perfect for the porch. Several of them we've put out on our porch. Depending on your space, you can pick one of these. https://www.charlestoncrafted.com/diy-plant-stands/
If you are thinking about creating a garden on your porch consider using pots to make a garden. I have grown strawberries in large planters by my back door.