Doesn´t masking tape do the same job? I always run a line or two around my harps, esp. the LO´s, and come away with the impression that they´re a bit "duller", in a good sense.
I think I remember harpwrench recommending using black electricians tape on the inside of the covers to achieve the same thing and it still looks good.
I definitely agree that holes in the sides of coverplates will make it much more difficult to get a full cup, whether acoustic or amplified. Electrical tape works fine to seal the holes as well.
While you have the tape out, a good way to see how well you're cupping is to seal off the entire rear of the harp with it. The sound pressure escapes out of those holes, AS WELL AS the uncovered holes on the front of the harp. Learn to play until you can't get sound out of the harp (or very, very little) when the tape covers the back. Then, remove the tape and try again. Any OTHER leaks you have are in your cup itself, again, whether acoustic or amplified. If you can't block all the holes on the front, block some with the tape to help identify where you're leaking, elsewhere in front or in the rear.
Your acoustic and amplified "WAH" will become much deeper when you master this - but don't expect to master it overnight! It takes most of us years. It is helpful, though, to know what the goal is!
