I've got my front fork seals on order from the stealership. Now I have to get my fork tool ready for when I tackle the job.

My understanding from reading the wiki and watching some youtube videos is that the front fork tool is a long rod with a 24mm hex bit at the end. In fact, this post suggests using a long ratchet extension with a 24mm hex bit socket on the end.

I found this tool on amazon. It looks like it might do the job, and it goes on a 3/8" extension which is much cheaper than the 1/2". My concern is that because the 19mm part protrudes from the 24mm part, it might not fit into the front fork piston.

To anyone that has worked on the front forks, is the socket end deep enough to accept this tool?

It won't fit.

I used two 24mm nuts jammed onto a bolt and held it with a deep socket and extension.

What year are the forks?  From what I've read, the damper rods were changed at some point and you would need a tapered dowel instead.  You could actually use a tapered dowel for both dampers but the older type that uses a hex tool is easier to deal with.

The forks are from a 1992 GS. Maybe I could cut off the 19mm part so it work just like hex bit?

Sure.  Cheaper than other options.