Fattening a base bar a mm or so

Those damn plastic Profile adapter shims…

Okay, so I need to get my aero-bars about 1 cm further apart. but the Profile alu base bar clamping area gets just a tiny bit narrower out there. If I try to clamp down the bars, the screws “bottom out”, and the bars just slide up and down if I apply any pressure to them. I cleaned everything with alcohol to ensure all the parts are clean and dry. I tried to thicken the base-bar with a little hockey tape, then electrical tape, but the shims on the clamps just don’t grip either of those.

Anyone have any suggestions? Maybe cut an old innertube and make a latex shim instead? I’m really trying to avoid just stretching my shoulders to fit the narrower bar position!

thanks