I like the mesh and the sparge pipe, nice construction

You said at the BBB it wouldn't take much pressure before leaking. Is that just the lid that fails to seal? I'm wondering if the top layer blocks up what happens - does the bio back up to the gravity feed container or leak out?
I quite like the idea of taking the lid off, scooping off the top layer and replacing, then carrying on.
There's nothing more frustrating with a tower that is pumped from bottom to top with accidentally sucking in a bit of HMPE or soap and knowing that will end up plugging the bottom of the tower.
Invariably even if I'm careful, whenever I change the tower's sawdust there is a plug of soap forming a bung at the bottom.