aerobars help

I bought some used profile design carbon stryke aerobars. They are in great condition but did not come w/ an adaptor (i think its called a shim) for smaller handlebars. Unfortunately, my handlebar is thinner than the tightest setting on the aerobar's clamp. So I put a wad of duct tape around my handlebars, enough so that the clamp will fit. The aerobars work now, but slightly slide up and down depending on my arm pressure. This makes it a little unsafe to ride w/ the aerobars. Anyone have any recommendations about where/if I could get an adaptor for my aerobars, or does anyone know any other ways to make up for a lack of an adaptor.

Also, is it common for aerobars to slightly move depending on where you put the pressure, or are they supposed to stay completely still? I have no experience w/ aerobars.

Any help would be appreciated