Beleive me, unless it is a all acoustic gig, you will need it.
Been there, and even in a rehearsal with PA for vocals, you will not be able to play the bass if you do not cover that chamber hole.
It is meant to be used on Kala U-Bass, so will not fit Harley Benton Ukulele Bass.
In my personal opinion, it could have a cork-like cone-shaped bottom, which would help insulating even more the chamber sound from the external environment, and still help it fit other brands as well, but it is enough insulating as is now.
It appears to lack any other option in the market anyway, so buy this one, even if it is not a Kala U-Bass. You can easily glue it with silicon gel without damaging the wood or the varnish.