Sanding drywall is messy. Super messy... and the dust is not only really damaging, but apt to clog a shop-vac filter in no time. There's a neat product on the market called Sand-n-Kleen which traps dust before it gets to the vac's filter, but it's not available locally (and I needed it that day)...
So, not able to find one at any hardware store, I decided to make my own using a Homer-bucket and top ($7), 2 vac-fittings ($8), 2 couplings which secured the vac-fittings to the top ($3) and PVC pipe ($1)... and it works really well capturing nearly 100% of the dust sucked into the water-filter.
The first variant found me with the intake pipe below the water line for a true water-bong effect, but in reviewing the original, it seems the pipe could sit slightly above the water line shooting the dust into the water and thus capturing it. I haven't tried this variant yet since the previous worked well. With the true "bong" the process is a bit more tumultuous — what with the bubbling water and all — and some water (about a cup or so per hour of sanding) does make it to the shop-vac... which is not really a big deal since it is wet-dry anyway.
Better yet — and even better than the original Sand-n-Kleen design, I was able to hook the vac line to the dust port of my Ryobi 5in orbital sander which made sanding a snap.
*Note, in the prototype, I fitted a drain at the top thinking I might have to glue the top to the bucket for an air-seal... but the Homer bucket and top are airtight under load, so it isn't/wasn't necessary. Also note, the original isn't that expensive ($40) and does come with an extended vac-line, so if you have the time to order it it might not be a bad idea, but if you're desperate for time or desperate to save $20, the home-made variety is an option. Worked for us.