I have been moving forward on this. I love the model-specific 3-D printed solution, but I am not patient enough
I went in for one of the tools MoT suggested. I figured: yes, it is cheaply made, but among the cheap alternatives available to me its simplicity is an advantage. My judgment was rewarded when, after it arrived today, I realized it needed lock washers to keep the d***ned screw tops from coming loose. Home Depot had me covered for under $1:
UPDATE: Just a follow-up. While trying to open the watch, my tool, above, broke near the end (away from the handle) where the curve connects to the straight piece. In retrospect I wasn't smart about it: I had put one of the two pins at the very end. I should have guessed that it would be weak there. Anyhow, the G-Shock won round 1.
The seller was apologetic, offered to replace the tool, but agreed to refund my purchase. At the hardware store I picked up some adhesive and a couple tiny hose clamps. I just wanted the adhesive to hold the break together so it didn't move around. I put a hose clamp over each curved section to provide support. Here is the result:
I confess that I enjoy trying to do the most I can with the little I have. So, yes, I used a big pipe wrench as a makeshift bench vice. Over time I do look forward to owning more suitable tools. I wish I could say I didn't scratch the case back -- I should have used MoT's tape.
I was feeling impatient, which as noted above, is bad. I used the tape before and I think it made the pins less slippery as well as less scratchy. Onward and upward.