Here are a couple of pics of the padlock setup .... you would have to adapt it to your observatory as you have all steel frame and different tracks to me .... you may just need to use latch fittings as I have used for the ends. I used off the shelf parts from Bunnings that could be assembled and disassembled as I knew I would be moving house soon after I built it. There are other photos of the observatory on my picasaweb site .... the link is in my signature.
I hope this helps.
Cheers
