This is how the Tube looks:
When I got the tube, it was not perfectly round. The difference in the X and Y diameter was 0.7 mm. This can be a problem because the cap must be turned smaller. The smaller the cap, the bigger the gap. And 40 meters down in the water, you don't want a gap!!
So, I "simply" pressed the tube round. With two wooden palings of 3 meters and two planks I made a squeezer. The tube was at 30 cm from the hinge and I was standing at the end of the palings Slowly the tube turned rounder, a tenth of a milimeter at a time. This must be done very carefully, because the point where aluminium starts to deform and the point where it breaks are very close.
Now the difference between the X and Y diameter is 0.2 mm. I think this is good enough.