I have the same telescope. No issues with the original mount. Just be aware that you are holding a delicate optical device of significant value with only a thin piece of ABS plastic. It will get very brittle in cold weather and could easily crack loose if bumped. It would be much better to use some properly sized scope rings to dovetail mount. My two cents.
Measure twice, drill once. ;) Measure withe the beak/front of the calliper the short distance between the treads. Then with the back side in the threaded holes the larger distance. Then the average of both distances should be close to the spacing between the threaded holes.
Don't have two threaded holes on the same screw thread. It will never be tight. Drill out the first to make a clearance diameter, then it can pull down and tighten properly.
You should have drilled Out bOth of the hOles in order to gain the extra clearance that was required there, plus you also needed to clear out both threads in the bar too in the dovetail bar in order to remove the threads from the hOles as having two adjoining threaded holes one in the dovetail plus the one that you are screwing into means that you can not lock the two pieces together as the threads will not align properly, for when you thread the locking bolts through they will not engage and pull tight, that is a pretty basic fundamental mistake people whom have a lack of training & do not possess the required knowledge in the field of precision or basic mechanical engineering often make. Getting a job done correctly takes a gOOd deal of skill so to get things right the first time takes real skill.
iOptron CubePro Mount Head $509.99 CAD. I have only had 2 opportunities to try it out. It seems to be very accurate, repeatable and able to handle the weight of the ETX125. I had to by a 45deg eye piece adapter to get a comfortable viewing angle. I think it will work out ok!
