On the first unit I ordered a new idler rubber ring for the spool, so what I will probably do is swap the two idler wheels.
If you look at the image, the one on the bottom has a new rubber tire, the one on the top is the Unit2's idler tire turned inside out. Both seem to have a fair amount of grip but the one on the top is much shinier.
But i agree, worst case, maybe rough up the plastic a bit for better gripping.
