Slowly, things are coming together. I created long clamps that will connect the carbon tubes to each other and to their power supply:

combined:

Each resistor will end up between two screws. I have two of the clamps as shown above, and one long one:

• The calcium silicate plate met my router and has two slots now. I also added holes for the clamping screws. One of the corners broke away when I drilled them, but I'll just glue it on again when I do the silicone around it.
• The cables are 2,5mm² glass silk insulated
• The short clamps one one side allow me to connect all resistors in parallel, or two groups of five in series. I'm not yet sure which configuration I'll end up using, so it's good to have both.
• Added holes to the frame for the cables, and rubber protectors. I'm not sure how hot they will get: they are close to the clamps, but also close to the surrounding air. I'll give them a 50% chance of survival.