OK, some things didn’ t work as planned. One of these were the rings I made (previously discussed here.). Despite the fact these were filled with chains, they bent and lost their shape while doing dips. A partial solution was to fill them up with sand (while sealing the ends with a glue gun.)
But even then, this was not still enough for the tubes to retain their shape, so my cheap-ass solution is to put a piece of pool noodle on each when I do dips on them. This is not necessary for pull/chin ups, though.
I would not call this a failed experiment since I can still use this contraption for dips, pull/chin ups and other exercises. It is, perhaps, the easiest way to make a pair of rings.
However, I do not believe I will build one like this again. The effort is not that much, but the amount of tools you need is a bit too much. You need a glue gun to begin with, and I believe the best material for filling them would be concrete rather than just sand.
I will keep using them since they serve their purpose. In the meantime, however, I’m actually working on an improved version using metal pipes, plumbing elbows and a 3/4 natural-fiber rope with a 600-lbs load capacity.