I agree that your loops are probably better able to withstand sideways impacts, compared to the Crown which did not have any "give".
The concern I have is that you drilled the holes....They were not molded into the case by the manufacturer. Drilling would seem to pose risks of future stress cracks, since the hole is not perfectly smooth.
If you want to add more strength, I would recommend adding a flat steel bar between the 2 washers. And then putting some rubber between that bar and the case. This could replace your rubber washers. So you would have, from top to bottom, the plastic case, a long piece of rubber under it, a flat metal bar, and your screws. The rubber would be sandwiched between the case and the flat metal bar. So that when you pick up the boombox, the load is better distributed inside the case. That is what Crown did. The only reason it broke is because the rollbar was smashed in a downward direction, which you can't do much about. But at least with an internal flat bar you could protect it from forces in other directions.