By full range, do you mean single driver, or full range multi way?
Building a speaker can be just about as easy as a sub, or as complex as a NASA science experiment. Diysoundgroup has various kits at various prices, but you'll still have to solder a cross over. That's usually the trickiest part for someone asking this type of question. Doable though if you can follow directions.
If you don't need a kit, you still need to find a design that suits your needs. And you'll be a little more on your own.
You can design your own, but be prepared for about 5 years of learning and experimenting before you try.