Thanks Paul for all the great idea. After many revisions, here is the most current version:
The feeder is anchored down by the base to avoid floating away or getting blown around by the reef tank water current. Just a push and a twist on the feeding tube, and the feeder can be removed from the under-sand-anchor:
After pouring in the BBS, you can unscrew the feeding tube from the feeder. The rigid air tube is compression fitted to the screw-like connector. You can put in a blind-plug to close the hole without getting your hand wet.

The screw-like connector is loosely fitted until the last turn.
Here is the top side with the funnel. I have some longer rigid air tube on order. I'm just using the short rigid air tube from my phyto culture for fitting/testing purpose.
Here is the under-sand-base and the feeder:
Everything is modular design. If you have a thinner/thicker air-tube, just need to re-print the screw-like compression connector. If you have a different size funnel, just re-print the funnel-airtube connector.
The screens are changeable as well. I know these holes are too big. I'll print some with tighter holes later. These are just the few prototype screens I printed for fitting purpose. They print faster with less dense print out.
The next step is to make the under-sand-mounting leg exchangeable, so that it could be buried in various places and situations.
Now, I need to get some fish to feed. lol