The tin box came with the boards directly from Henning (DF9IC), while the copper heat spreader is from G4dbn. I can't remember the cost for the heat spreader, but it was quite reasonable, BUT I bought one that didn't have the holes drilled/tapped. And I found that drilling and tapping 12mm thick copper plate with hand tools isn't easy! If you have the option just buy the pre-drilled/tapped version....
By the way - some words from Henning regarding the large (2w) resistor feeding the bias, and on how to tune the PA -
"value not criticial, depends also on the voltage you want to apply there for bias. I use 470 Ohm / 2 W which works for both 12 and 28 V. The 5 V Zener must reach its voltage, with 2 x 1k load from the pots (plus a bit more from the 2k2 at the gates). For 12 V a value of 470 Ohm is minimum, for 28 V you can go up to 2k2. The required gate voltage for about 0.5....1 A bias current should be a bit more than 3 V.
For the tuning process it is enough to disconnect the drain voltage of the unused transistor. Start with 5 W drive, tune one transistor (expect 50....70 W at the output), same for the second, then connect both and you should be close to 250 W, you may try fine optimization. Each transistor should be driven to 10 A @ 28 V. I test each pallet up to 32 V, and use mine with 30 V supply."
It's also a good idea to use a circulator on the input of the PA to protect your transceiver when you're tuning the PA!