You'll get a better spray (smaller droplets, more range) with .020", but I've read that apertures below about .030" tend to clog.
Crude method: lengthwise thick-walled alum cylinder with pressure feed at either (or both) end(s), OD small enough to miss the flying bits, ID is distribution. Tube can pass out of both ends of valve cover, thread the ends and use Stat-O-Seals and nuts for leakage.
Drill 1/32" holes as needed; the tube can be rotated to clock the spray where you want it. If the X-area of the tube is small, use smaller holes closer to the pressure feed.
Important: use a fixture for drilling, the hole edges must be sharp and identical.
