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LED pixel boxes. Not retro, but something I've oft considered building myself!

WHOAH there! I've just clicked the link on the resistor you originally suggested, Wayne. 100m Ohms.. That's 0.1 Ohms. One hundred milliohms. Effectively a short circuit. The peak current through...

Right!

Looked at the datasheet for this series of resistors - the HS25 series would need a 1mm thick heatsink 535sq cm in size for the power rating of 25W.

These fellers.....

Something like this: https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0159590/

Just one per channel, per controller should be enough. Be sure to mount them on a heat conducting surface,...