I got bored one day and tore some paneling off the garage wall and started making knee hinges out of it. For the center hub I'm using 7/8" chair feet caps cut down. One inch snap rings fit on them perfectly. I ran four screws and nuts all the way through both plates, which are also glued together. The center screw also has a large washer. The outer perimeter of it is still visible when the chair cap and snap ring are in place. I decided to omit the chair cap and snap ring on the two inner knee hinges for clearance purposes and because you wouldn't be able to really see them anyhow.

The hinges will have 1/4" strips attached to them at right angles at the top making them basically "L brackets". A recess is routered into the top of each knee to accomodate this. This way you get a nice flush fit with the lower knee plate without having to cut a groove and it also allows for complete painting of the knee plate, knee and hinge before assembly with no need to touch up after assembly. You can only see the 1/4" strip if you lay on the floor and look up at it from underneath, otherwise the mounting strip is hidden by the upper knee plate.