I worked on my first gasser this year as well and got some great feedback from some of the guys helping you now. The picture below is how I did mine. You didn't mention what kind of plane it is, so I don't know what kind of room you have. I have a Rascal 110 so there is plenty of room behind the firewall, though it's not real easy to get to, but not under the cowl. The tray that's in the picture has a screw through the firewall holding one end and the other end is screwed into a block that I CA'd in at the back.