The ones pictured in the link above look like mine. They basically have the consistency of a pencil eraser. For that matter, you could also probably use a larger block-type pencil eraser instead of specific toilet bumpers. I'm not sure how easy it would be to drill a hole in them, but it might work. You'd need to not only drill a hole for the screw, but you'll also need to countersink it as the screw obviously needs to be completely recessed. But like I said, I can pick some up for you if you'd like. Not really worth alot of messing around with.
The hard plastic ones I linked to in the other thread (for the reference of others, post #2 in
this thread) had some screw housings that protruded below the bumper, which would have had to fit into a countersunk hole in the seat. You do not want those.
Dave