Here I am setting up the other cap to weld to the VBT pipe itself.  This is a standard single V weld and I set a 2mm gap up with some washers.  Then I tacked the cap, removed the washers and completed the root run all the way round.

The last picture shows the inside.  The bead should just come through to ensure 100% penetration but not be to much or to lumpy so it might restrict liquid flow.  Looks ok to me.

Submersible Build Log