When I installed one in my RV-10, I treated it just like the plans did for the fuel line, except I used steel fittings. I don't think I have any photos of this on my build site.
I have what you are talking about. Return line was placed right next the feed line. The return line is 5/16 so the firewall hole is slightly smaller. I don't recall the exact size. This location of holes is working out well but you have to measure carefully. Basically it is as high as the heat plenum/valve will allow.
I spent a lot of time laying out the exact locations of all things on the firewall so that all would be logical and have adequate separation before I made holes. I have electrical on the left side, and fuel/oil on the right side. The return line firewall pass through and the start contactor is the closest place these two system get. I don't have a picture of the front side unfortunately. Obviously the return line is closest to the starter contacter lug, and as I recall there is about 1.5-2.0 inches between. Enough to get a wrench on either.