This is something I came upon today. It's for an electric bird but I know you sometimes have to build a box mount to extend you engines out too, so it may work for you.
I'm holding the mount on a nice slippery glassed firewall, trying to mark the hole locations for the mount bolts. Well that didn't go so well. With the plane in a cradle, it would slip all over the place while I was trying to hold the mount with one hand and mark with the other.

Well after a half dozen or so tries. I sat down the mount and started thinking of how I could do this. Took the wing off, and tried holding the bird between my knees and again the mount would just slide all over the place as I tried to square it on the crosshairs and mark the holes at the same time.

"Now what?" I say to myself as I look at the mount and the firewall. Hmmmmm, there's already a center hole in both the firewall and the mount. BINGO!!!!!!

I go over to my hardward cabinet and dig out a toggle bolt, you know the bolt with a nut on it that has a pair of spring loaded wings on it.

Unscrew the nut, insert the bolt through the mount, reinstall the nut and shove it home thru the hole in the firewall. Tighten up the bolt enough to snug the mount, line up the crosshairs on the firewall and tighten down the bolt.

Rock solid and not moving now. Take my drill and mark each hole, no need to try and use a marker or center punch as I already have the mount acting as a guide.
Unbolt everything and finish drilling the holes.

Son of a gun a perfect fit.