I see what you mean about wishing for a lathe. Until I can find a good way to file things level for bolting
lids on, keeping sawn-off-and-siliconed PVC sections watertight will be a major part of maintenance.

On static divers, what kinds of ballast systems are available for a single on/off channel? Does it
have to be an always-pumping/always-blowing mechanism, or can it have an off position ?

"Blow negative to the mark !"