No matter what you do you are going to have to be able to at least modify the oxygen sensor output to the computer. If not it will readjust the engine back as close to the factory settings as possible or it will go into poor mileage limp home mode.
Once you get a box that will let you lean out the engine; your options expand, water, steam and vapor injection are proven to increase mileage and are relatively easy to home brew at little cost. (I got a 24% improvement (daily driving) with simple water injection (think large hookah) during the last gasoline hoax (shortage) but I had to remember to refill the water reservoir every time that I got gasoline (It would not run without the water). Resign yourself to building/experimenting yourself, most if not all of the mileage increasing devices on the market are overpriced frauds, IMO.
BTW, all mods of this kind are illegal and will cause your engine to fail at least the visual portions of a pollution test.