For anybody who may be interested in such things, I bought a handful (direct from China, where else ?) of stainless disks for very little money.

On receipt, I found these to have very rough edges where they've been stamped out - but a few minutes work with an abrasive pad made them quite acceptable.

Not so acceptable are the claims being made that these have 4 settings: Open, Closed, QX and ventilation. In practice they have three, as the 'hole-array' and 'slot array' perform exactly the same function (unless you happen to have sub-3mm honey-bees of course ...):

However, these disks are made from magnetic stainless-steel, and so a full closure could very easily be made over the 'slot-array' with something attached to a magnet - if you should ever need such a function. I'll probably only ever use these myself in either fully open or closed (vented) mode.

At 4.5mm the QX slot is a little on the large side for natural-sized bees, but maybe it would work ok ?