Update November 16th, 2018

Adding pictures to the bottom of the old log doesn't work (like usual, picture-handling is a big fk'n mess here at hackaday.io). So here's the update:

I found a fitting piece of 1.5mm thick aluminium, drilled a hole and cut a 75mm diameter hole with my jigsaw.

The rosette is mounted with twelve cup rivets.

Stick-out of the stove pipe is around 60mm.

Mounted to the oven with cup-rivets. It's sturdy as hell. You can see the longer stove pipe just shoved down onto the stick-out pipe. Fits like a glove, happy with it!

Most of the year the oven won't be used, so I decided to not fixate the whole stove pipe to the oven.

Those thin, aluminized steel pipes are tapered. So I cut a 250mm piece from the 1000mm pipe. This piece is mounted into the oven and the long piece is just shoved onto it when I use the oven. Nice!

For the fastening to the oven I used a piece of left over aluminium sheet and printed a drill/cut guide which was glued on with PVA/wood glue:

The tabs are then bent by 90°: