Down here, all fence boards are rough sawn, just to make them affordable.
I can offer you a coupe of things?
Any asphalt product like Creosote Emulsion, Aspahlt sealer etc. will preserve the wood as well as anything else, and be much cheaper too. Using a few of these boards, set them up on sawhorses, and make a "tray" to set a board in by lining the tray with cheap plastic. USe a pump style garden sprayer, and wet it down with the sprayer, then with a paint roller, roll it on to make good contact. The roller at first will be dry, and want to pick up the liquid, after a while its wet, and will be great at ensureing you have introduced the liquid to the wood evenly. which is the point.