Splat commissioned me to make this as part of the rail system earlier this evening.
Half done in creative mode for planning purposes, then tore most of it down after realising it was wrong (except for the frame). I then finished it in survival mode even though it took bloody ages to collect and smelt all that clay. It is very plentiful on ocean floors now though, and you can use the bucket exploit to stay down there as for as long as you want (just don't drown like I did, it's hard to locate your stuff again).