Charles,
Yes, cutting away the corners of the lockers is very important in shedding the water quickly. It is not quite the same issue as that to which Claus was referring, which is preventing water getting in when heeled for long periods. But when leveled, it makes a big difference to the speed with which the locker gutters drain.
Below are three photos which should help.
The best tool to use is a jig saw, with several new metal blades at the ready, as it will blunt them.
Once cut you can sand, and if you wish, paint the cut edge. Epoxy, polyester flow coat, or a decent paint can be used, but given they should not be permanently wet, there is no need other than aesthetic to do this.
Best wishes,
Matt
