I did some searching and at least in my neck of the woods it seems that such blocks can be ordered and delivered. Maybe you don't need to DIY and you can just buy what you need?
Can you justify a few hundred for a big chest freezer and a cooler? Then you can use the cooler method described in this link or in one of the other comments from ir77 on this thread. But just put the cooler inside the big chest freezer.
It is also terribly slow. I was thinking about something like a cold plate on top of which there is pot of water with insulated sides that freezes from the bottom up - and something to move the water around. so you get constant remelt of the top layer so no impurities.
A second hand ice cream maker could provide the cooling power.
I need big blocks, more than my freezer can cope, but not enough to justify couple of thousand EUR worth of machine.