Rear xmember mounts/ paddling pools

I recently welded in some new ones and have noticed whilst the rear bumper is off that they are holding a lot of water.
Have been considering drilling a hole in each mount to act as a drain but not sure if this will cause structural issues for MOT. Anyone else done this?
Have already given them a good coat of hammerite and will be smothering them in waxoil once the rain stops..
Thanks