There are propulsion test stand plans here.
I built mine smaller than dimensions on plans for small motors.
I also built this balance.
I use paper of known density (printer paper) to calibrate.
I guess I should get some standard weights for calibration.
Gordon Johnson uses a similar propulsion test stand which is simpler. He only uses the peice with the L on a pivot.
The one I made has a string on top of the L that goes to another part that has a pivot point which holds the motor. I think Gordon's version will have less room for error. The only critical dimension in the test stand design is that the L should have both the vertical part and horizointal part the same length. I have seen designs for test stands that use deflection of a rod to measure thrust and also ones that you connect a string to the tail of a plane to get thrust.
Nice job on the LRK.