something else I have tried is:

on my suky and extra, I cut a piece of ply that goes over the spar and an anti rotation pin. I placed it on the aircraft and measured with a plumb where the desired cg was supposed to be and drilled a hole and a few holes either side.

then put it all together with the ply wedged in and picked it up by a string atached to the middle hole to check CG. it should hang fairly level. I did my 25 pounder that way recently. then when I wanted to move the cg back 1/4 inch, just hung it from the next hole back and moved the batts to get it.

pretty simple if you dont have scales or a mount.