Well, I decided I needed a dummy load. I ordered a bunch of 3w @ 510 Ω to put in parallel, so roughly 30W handling (perhaps more with the added cooling for short periods).
So, 10 of them soldered up to some scraps of vero, and a reading of 50.6 ohms taken, so all is good on that front.
I added the so236 to the tin lid, and solder/welded it all up. It will be sitting in a Golden Syrup tin, with a 500+ml of mineral oil. I had to silicone around the underside of the so239 to prevent the oil from oozing up through the connector.
Ah, job done, and a total cost of less than £8 🙂