I almost resorted to a C-clamp I use for woodworking but thought the steel would wreck the crystal.

This is how I put all of my Timex casebacks back on - with a wood-clamp. Put a 1/4" piece of wood above the watch (you can put a wash cloth between the crystal and the wood) and one below, on the caseback and slowly turn. It will pop back on with a satisfying click.

I use this style of clamp:

You'll find that a crystal tool (that also doubles as a caseback tool) works in exactly the same way.

 

I avoid them for this very reason. In many instances it’s almost impossible to remove them without doing some sort of damage to the case back. Even minor damage is still damage

 

I had a watch just like this. I didn't want to risk damaging it so simply sold it.

 

There are tools design for such things, but I use a knife blade to open them and a crystal press from harbor Freight (the blue gizmo picture below) to close them. No sweat. Just use the correct size die.

 

They can be a PITA (as you found out), but the right tool can make the job easier.

This is why I have esslinger.com on speed dial.......

 

Snap back cases are usually easy to open and close when they are Seiko or Citizen watches that retailed for a few hundred. Using a large case holder and case knife usually works well. Snapping the cover back on can be done with finger/thumb pressure using both hands/paws. The sub $50 or so watches can be a real challenge.. Usually use a mallet on the case knife or flat driver. Closing the case requires dies that will clear the crystal and mount/cup the case back. Application of a good deal of pressure using both hands/paws is needed for a handheld caseback/crystal tool.

I'm kind of surprised I never broke a crystal.

 

I had the same problem first removing it and then ofcourse putting it back. I had small knife kind of thing that came with a watch tool kit but it never worked i damaged the case back with scratches (the sharp end is bigger than slot that would allow the tool to get in between) . I tired what other suggested a pocket knife i ended up getting a cut on my fingers also more damage to the case back. It will be very tough to get it off with out causing at least some minor damage. Later i used a small precision flat head screw driver and it worked like charm with almost (...almost ) no damage. While putting it back i used simple watch crystal tool by putting a cloth on the crystal face it worked multiple times both these tools are really inexpensive and will come in use later as well on other watches.