Conclusions

The design is servicable, but should it jam and plastic enter the concrete section it gums up and is very hard to remove. The high temperature capabilities of the concrete can be exploited here. First remove as much as possible with some form of ramrod - I used a bamboo skewer - then heat a metal rod (classic Kiwi No. 8 wire is good) to dull red and ream out the unwanted plastic with it.

The specimen produced here is probably a bit too long. Approximately 20mm could be shaved off, reducing the risk of jamming. It is possible that a silicone baking paper lining might reduce the gumming up issues and this will be tested later.

The bolts that connect the concrete to the poly-holder should be held on with 2 nuts each. This will make it possible to remove the concrete from the poly-holder; a flaw with the model described above is that I can't get the darned thing off!