I’ve worked with the Keystone holders and can confirm that those will hold protected cells, at least some I got from nkon.nl

Can you please specify the exact type of these cells.

A friend found a 68.8mm cell that fits; the Keystone holder caps out at 68.88mm. Most protected cells are 69.2mm…69.8mm.

That's a good tip for safety conscious visitors because the webshop is in the same country. Probably a good idea to put it on the event wiki. Is the event still going on?

The last day will be this Tuesday.