Good quality! 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.
This was a risky purchase on my part due to the fact that I was not buying it for a Kala U-Bass (for which it is specifically manufactured.) I was, in fact, buying it for use on my Sigma TM-12E, a half-size guitar. As advertised the product was slightly too small for the sound hole on my guitar. However, this was an anticipated and accepted shortfalling. It took a few roles of tape around the circumference of the Feedback Freezer to bulk it out just enough to sit in the sound hole of my guitar. In terms of its use, I have no complaints. I had not experienced feedback problems before and this product was bought just as a precautionary based on a recommendation. However, I cannot fault the product to date.