Robert
I made the vertical wood piece 4 3/4" long, the horizontal was 2". This put the channel in the center of the wood spacer, and the length was enough to allow a straight shot with a hex wrench. It was also able to fit inside the cowl without shaving corners. What I meant with the channel height was to try to make as much of the standoff dimension come from the channel piece instead of the wood spacer. You will need to get around a total of around a 2" standoff from the wood and channel combined, using the OS 160. A smaller wood piece shortens the bolts needed to mount the wood spacer to the firewall. You can glue them instead, but I made mine removable. The problem with getting a deeper channel dimension is that with the limited stock that home centers carry, when you go up in size, the channel wall gets thicker too, and it gets out of proportion for our use. Taller channel sections with thin wall sections might be available from scrap pieces from aluminum storm window sections.