This is definitely something to take care of before riveting the aft fuselage floor in place. In fact, for anyone reading this, an even better time to do it would be before the turtle deck is even installed!
Anyway, I will need an ELT eventually, but the plans leave you on your own for mounting it. After a bit of research I decided I'd plan on using the ACK E-04 ELT. However, ELT's and especially their batteries have a finite shelf life, and who knows when this plane will actually fly. So instead of buying the whole ELT now I just bought the mounting tray that goes with it so I can figure out the mount now, and just strap the ELT to it later when it's closer to flying time.
I did what I've seen many others do, and planned to build a mount on top of the right upper longeron, behind the baggage compartment (in front of the aft seat belt mounting point). This is a sturdy spot and still accessible for now. I only riveted the aft floor around the access panel so far, so I was able to bend the forward half down and get a decent amount of room to work.
I started by just cutting out an arbitrarily sized piece of scrap sheet, with the angle of the aft fuselage side cut on one side of it (about 10 degrees).
Next I drilled that piece to the longeron and upper cross tie. I added two pieces of scrap angle I had laying around as well. This was an okay start.
The mounting instructions for the ELT said it should not be able to move more than a tenth of an inch in any direction with 100 lbs of force applied. I don't have a great way to measure that, but it seems pretty rigid. From here I just pushed and pulled on it and examined where the mount seemed most flexible, and added reinforcements bit by bit until I couldn't really budge anything anymore.
I added two washers per screw to raise the tray up a bit, which will allow the mounting straps to clear the 1/8" thick reinforcement strip that I added to the top of my mounting sheet.
In total, what I added to the plane was 11 ounces. Not too crazy, but I probably could have made it lighter with some less haphazard engineering I guess.
Here's what I ended up installing.
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