For oystershell thats a very very big container.
Size is relative to the flock size, my oyster shell (and grit) feeder is very similar size and design and I have to fill them every 2 weeks, so I don't think it's that large at all at least not in my case...
When building one it's much easier to simply move the vertical pipe to the side of the cap on the bottom and screw through the side of the cap into the pipe, this also allows for you to more easily secure it to the wall...
I secured mine to the wall with shelf keyhole brackets...