The Watson Gym economy glute ham raise is fairly cheap to buy. Can anyone think of any drawbacks to the flat pad?
That sounds like it would be a natural GHR rather than an actual GHR. The rounded pad seems pretty essential to the movement. You could try to build your own round pad to affix to it.
Usually your knees will hit the sloped part of a regular pad, not the apex:
(Note how the knees actually come off the pad at the bottom of the movement)
This is not possible with a flat pad.