This may give you the idea of building your own (cheap) dummy load and do your tuning through this.
You can subsitute the shown value components to handle your mega-watt amplifiers.
For example, you can use twenty, 1K (1000-ohm), 3W or 5W resistors connected in parrellel.

The components shown here are two 100-Ohm, 5W resistors connected in parallel and submerged in a tin of oil.
This will more than handle the few Watts output from the transmitters that QRP'ers prefer.

Start by drilling a hole hole in the lid of a 250ml paint tin to suit the SO259 connector. Solder around it to prevent any leakage when the oil is added. Next, a leg of each resistor should be soldered to the metal lid and the opposite ends both/all soldered to a length of stout copper wire which is taken to the center connector of the socket.

With a multimeter, measure across the SO259. There should be a reading of 50-Ohm's and no open/short circuits.
If all is well, fill the tin to the brim with engine oil and attach the lid firmly in place.
At this point you'll probably notice that oil will splurge over the sides so don't do this on the kitchen table!

If you don't fancy the mess of oil it's also possible to fill the tin using sand. This will also dissipate
any heat generated by the resistors. I've not tried this method but there's no reason why it shouldn't work