You can use a sort of "double diameter washer". I've made that name up, but I am pretty sure such a device exists:

![enter image description here][1]

That's a cross section of one of your linkages. The white things are the aluminum pieces, the grey one is the screw, the yellow is the nut while the red is the "double diameter washer". If you can found some washer that is slightly longer than your aluminum bars thikness you are good to go. Of course if it's too long the linkage may become lousy, but with a good cutter you can trim them down, or you can also add some regular washers between the red one and the nut. [1]: https://i.sstatic.net/mEKcj.png

You can use a sort of "double diameter washer". I've made that name up, but I am pretty sure such a device exists:

That's a cross section of one of your linkages. The white things are the aluminum pieces, the grey one is the screw, the yellow is the nut while the red is the "double diameter washer". If you can found some washer that is slightly longer than your aluminum bars thikness you are good to go. Of course if it's too long the linkage may become lousy, but with a good cutter you can trim them down, or you can also add some regular washers between the red one and the nut.