I made myself a corner scale yesterday.
Using the area of the piston you can work out the Display PSI>Actual Pounds>Actual Kilos. Each PSI segment on this one is 6.4kg and you can easilly read 1/2 segments, so a ~3kg resolution upto 320kg.
Total cost was about £30 including 1/4" BSP taps that I can use for other projects. I could make another 3 of these for £20 each and do all the corner weights all at the same time, and even use the jacks to level off before reading.
Cheers
Stot