After a little playing about I got it to seal air-tight (so I could blow into the inlet with the outlet blocked without a hiss of air). Good to go!
Charging with woodshavings.

In action:

(I ended up soldering a 90 bend on the outlet to stop it splashing out of the drum)
However, very quickly the flow started to reduce - way before the sawdust would normally be spent. Not to worry, I though, it's sealed OK and the pump can push a little pressure. Bear in mind the pump only does 4.5lpm or so.
There were a few alarming *boing* sounds as the top and bottom dished out under pressure... but that was OK as it was all still sealed at that point, no leaks.
Then it started to leak by the clamp handle. I moved it around and the leak followed where the clamp handle was. So, I though, I'll just squish the clamp in the vice a bit so it squeezes the lid down a little tighter. What I should've done it just do the bit by the clamp, not the whole circle.
Consequently it was rather a tight fit, and I've only gone and sheered off the tag of metal that pulls on the clamp ring

Will have a crack at welding it but I'm not hopeful for success (it's tiny).
Anyway, I made a right mess doing all of this.
Here's how the top dished out under pressure - the bottom is the same.

I wonder if the clamps on the drums max.print has are compatible?