Countersink Inside a tube

[editor@glue-it.com]

There are tools made to countersink the otherwise inaccessible back side of a hole but such tools would have been expensive overkill for your problem.

Your solution of constructing the needed tool inside the obstruction is an ingenious technique that should be in everyone's mental quiver of unusual solutions. Good on you.

Thanks, the thing is when you need a tool like this you need it fairly urgently and hence making it was a good option

Cheers, Nigel