re: chain rub, sorry I missed it, another option for more clearance in your situation to keep about the same ratio is a smaller front sprocket and smaller rear sprocket. (or just a smaller front or rear if you want to tweak the ratio).
Also like the cutting board idea, cam chain tensioners been using chunks of nylon for ages. cut a strip, heat it, bend it around the tube, screw it down. (honestly the skateboard wheel looks weak, on many levels)
