You would not make this of plastic or wood, running unattended unless you want to risk drama if the fan stops or it falls over. A thermal cutoff might be a good idea. Or you could let the place burn and confirm why Dyson moved it all to Singapore.
In North America, steel ducting is commonplace at the local hardware stores, for home heating and gas vents, dryers etc. I'm not sure what the UK stores might have.
The resistors have a 400ppm/C tempco, so you will see the load current sag as they heat up.
They are rated for about 200C RISE at 17W under convection cooling, and let's say you achieve half that with forced air to 100C RISE to give 15.6 ohms I think and 6.15A 295W if that drop is OK. Otherwise, you would have to trim or add a rheostat, after temp stabilizes.
In reality the top resistors will run hotter as the lowers heat the air, and there is turbulence, so I would stagger the parts and have them cross the center of the duct.
You could look at using a mains-appliance like a room heater or toaster oven, stove/range element for the load.
I use the power resistors that can be mounted to an aluminium heatsink but this is expensive.