Holga negatives - flat - masked - ???
Nooo, nooo. Eschew a negative carrier altogether. Just push the strip into the gaping maw of the enlarger. Or as Bill suggests, make a carrier from ripped cardboard.
Now as for the enlarging lens, you need something like a plastic Isco lens, maybe a Fedar or Testrite. But even these have too many elements. I mean, how many elements does a Holga have? One, right. So it must be with the enlarging lens. So either remove the retaining rings from the lens and get rid of all those pesky extra elements (leave behind a positive one). Or just take a star drill and break out those superfluous surfaces - the scratches on the left-behind element will just add to the je ne sais quoi.
Hmmm ... maybe that third cup of espresso at breakfast was a mistake ...