I used a razor blade to remove the black rubber. You have to be careful not to cut any of the semi-transparent plastic. Only cut the black rubber.
As far as the length, it is the same length as stock. It gets fully inserted. No need for me to measure.
As far as it getting stuck and not being able to be removed, I have inserted and removed the adapter about 10 times so far. It fits perfectly, like it was made for the M500. If you can insert and remove the stock plug, then you can insert and remove the DIY adapter. It has the same dimensions.
As far as the semi-transparent plastic casing, yes it is thin and flimsy. That's why I said in my post above that "The only caveat is that they are somewhat fragile. They appear to be able to stand up to normal use, but if you’re overly rough with them or put a lot or stress on them they could break apart. However, if I break one who cares, as they only cost $1 each and take 30 seconds to remove the rubber covering."
I'd recommend to spend the $1 and get one of the adapters. Then you can check it out in person. When you can see it and hold it with your own hands, I think it would clear up any questions. Photos can only do it justice to a point.
Hi. Fully understand what you mean . Appreciate your sharing. Will definately give it a try when time permitted. Thanks again