To attach the engine control cables underneath the panel, I made this little bracket out of 2"x2"x1/8" aluminum angle. The three small holes are for the cables, and the other holes are just to lighten it a bit.
Because the bracket will be somewhat visible hanging down under the panel (at least when you're standing on the ground behind the wing looking in) I decided to paint it black. The paint was taking forever to dry in the cold garage, so I laid the bracket on top of my little electric radiator, using a piece of scrap alclad as a tray. It cured fully after just a couple hours in the sauna.
Here is the cable bracket clecoed to the throttle quadrant supports. The green cable is from Van's and the clevis ends are from Aircraft Spruce (the clevises that Van's sells won't work).
Another view. I had to have at least one cable on hand so I could know where to mount the bracket, but without mounting the bracket I wouldn't know what length of cables to get. I chose to solve this dilemma by buying one cable in a standard 48" length, and using it both to locate the bracket and to determine how long the real cables actually need to be. Since it's a standard length, I can return it for 90% credit later, and I don't have to waste money trial-fitting with expensive custom cables that can't be returned if I guess the wrong length.
I hooked my test cable up to each of the three engine controls in turn. Here's a view of how the prop cable is routed from the eyeball in the upper-left corner of the firewall, down to the prop governor bracket. With this part adjusted correctly, the portion of the cable inside the fuselage needs to be a few inches longer.
The throttle cable comes through the firewall low and in the middle, and runs up to the bracket I previously bolted to the sump.
Inside the fuselage, the throttle cable is about an inch too short:
I forgot to take a picture of the mixture cable, but you get the idea of how this process went. In the end, I decided I needed cables that were 49", 51", and 52" long. I called Van's to order these and found that they also have standard-length cables in 49.5" and 52.5" lengths… the cable lengths can be varied somewhat, so I decided to order those two standard lengths and one custom 51" cable.
By the way, these cables come with a most interesting label on them:
Not for use on aircraft indeed.