I have an oldish Patterson tank here that has the adjustable clipping section of the top spiral above the top spiral with the grooves underneath. This means you can push the two together as close as you want - it will even go down to Minox width. The working distance is governed by grooves at the appropriate places on the central tube. For 16mm I merely had to cut/file a groove on the central tube at the right place and it works fine. I know someone who has done this for the Advantix films he uses.

Most other tanks I've seen have the clipping mechanism between the spirals and it wouldn't be nearly so easy to change..