When I need a tool to work a TH tight in small cord, I do use a stout little scratch awl I have. I turned a conical fid for myself out of a oak dowel on a lathe. To say the least, THAT was a learning experience. My marlinspike, a very simple one, I bought online. I found it at McMaster-Carr.
www.mcmaster.comSearch for "Marlin Spike" on the site. They are at the bottom of a page of rope and goodies.
6 inch spikes are about $15. That was one of the best prices I could find when I was shopping for one. They are high-carbon steel, so rust can be an issue, but I like mine, and it has served my small to medium duties quite well. Convenitently, there is a lanyard attachment hole, which accepts a shackle quite nicely. I used the spike to tie it's own lanyard, then attached it to the tool with a stainless shackle I picked up cheap at a local hardare store.
I, for one, would love to see a post with what you end up finding.
Happy hunting!
-Abe