Closing the donut shape is tricky, You really need detachable fingers or the help of a dexterous mouse. As I say – tricky.
William H suggested using two sets of tabs on the final joint, this works well in some circumstances but unfortunately as the tube is an octogon in section it is collapsable. This makes it really hard to keep everything lined up as the glue is drying. So instead of that I've tried putting the tabs on the outside. To hide the tabs I've made them as large as the adjacent faces. That way they cover the faces completely with the edges lined up with the edges of the model.
In this model I've created a final section with these oversize tabs.
This piece is glued into place last. There is enough flexiblity in the model to let me glue the final section on fairly easily.
…then by gluing the flaps down I can close the circle.
In the final version I have four large tabs on one side and the alternate tabs on the other end.
It all works rather neatly. Okay – model done – download post to write up.