I used 1/8" birch ply with no reinforcing angle on the bottom. You certainly wouldn't want to put anything heavy back there so a strong floor isn't necessary. I am sure light aluminum would also be fine.
Per ideas from this forum, I am going to put a zipper, or probably a velcro opening in the back of my baggage sack to access the extended area for longer items like fishing poles, etc.
I think the fabric in the Kitfox is cordura. We bought and old sack from a KF 5 and extended it with fabric gotten off the web. We're using thin plywood as the floor to rest the baggage sack on. If you are good at sewing you could make your own. Here's a link to cordura fabric.
Thanks Tommy, sewing isn't my strong suite but I may be able to make some thing work. As for the link you supplied me, what is the difference between 30 and 1000 denier? Is that similar to cotton weight?
Also thanks for the info about the floor boards Jim. I guess 1/8th birch ply should do. Next time I fly home I'll try to measure up how much I need to order. I may do the sides with the birch ply as well instead of fabric. Something to think about. I do like the zipper idea for a number of reasons too.
If you do the cordura fabric go with the 1000 denier (heavier weight). Also go to sailrite.com, they have a video showing how to sew a zipper on a radius and sell oversize zippers. Not too hard to make a cargo bag, I did it.
I made an aluminum angle frame for my extended baggage area. It is made to fit the various tabs that were welded on the baggage area tubes.
The frame is then bolted to the tabs. Inside the frame I use two pieces of an aluminum honeycomb sheet (about 3/16 inch thick) that is cut in the shape of the angle iron frame.
The floor is made of two pieces (cut lengthwise) so I can easily remove them when it is time for my annual.
The weight of the whole assembly is about one pound heavier than the balsa honeycomb floor that came with my kit.
The honey comb is strong enough for me to stand on, which I often did when I had the turtle deck removed and was working on attaching the windshield.
I couldn't find a picture of the honeycomb but I have a picture of a balsa honeycomb that fits the frame I built but is used in another Series 7 with the extended baggage area.