Ok, definitly not legal. If you didn't mind the looks, a saddle tank off a pickup or a smaller tank from a car of some kind would do, but I assume it would have to be D.O.T approved for that purpose, otherwise I would even say a forklift tank.
A RV horizontal vapor tank can be frame rail mounted or bed mounted. Horizontal tanks are ASME approved which means they can be permanently mounted. They come in all sizes. A 10 x32 is 7.9 gallons and will fit in between the frame rails of the Ford if you want to mount it underneath. Larger diameters such as a 12 x 32 is about 10 gallons so if you go in the bed, you can use one of these.
You can use vertical style tanks with a vapor draw system. They are DOT approved, which simply means they can be transported full or to be filled. Think of every travel trailer/pop up. They have to be mounted upright and secured so that they can't tip over. Mounting them in a vented toolbox for example works well.