I would be very doubtful of getting a good VSWR at 2.4GHz by using resistors in parallel, at these frequencies dummy load resistors tend to be co-axial in nature. Have a look at this page http://millimeterwave.free.fr/ATT_DISPONIBLES.htm and see if anything takes your fancy, it really depends on if you can deal with 7/16 connectors. Also you will need more that 40 dB in total to not damage your power sensor. A quick look on Ebay (UK) and I see some 50 Ohm loads 25 watts for about £11, they will only be rated for intermittent use I would suggest, but they say they are good to 3GHz or even some resistors that can be bolted directly to a heatsink that will take a lot more power if cooled correctly, so please do have a look around various outlets before you decide.
I am probably guessing wrongly, but wonder if you will be trying to test a Pyrojoe 20 watt amp or similar, if you can give your self some margin in case it is 25 or 30 or even more Watts output.
What ever you do be careful at these frequencies and powers as you are taking microwave frequencies now and soft tissue damage is always a possibility.