Here's a easy to build, approximately 100w 50 ohm dummy load built by VU2POP Pratap using 2.2k ohms 2w carbon resistors.
This is what VU2POP reports about the dummy load he built:
"In 90s, I had built a cantenna type dummy load with appx 42 nos of 2.2k 2 watt resistors, built into a threptin biscuit tin.
After its assembled check total resistance and pull out or add as required
Works great , some years ago gave it to vu3tyg nitin, maybe he still has it,
I used it as dry since I was testing up to max 100 watts"
"I used it dry. Resistors are 2 watt and about 42 of them can handle aprox 80 watts.
I have used for less than a minute while alignment of tx, With oil it should handle double the power easily. Good engine oil should be ok."
A good quality transformer cooling oil is the best option to fill the can for heat sinking the resistors.