Question on Reverse Crayford Focusers

I am looking at JMI's Reverse Crayford Focusers (RCF) for my scope. It
looks like there is a small gap between the Focuser tube, and the body of
the focuser. Won't this allow stray light into the scope, damaging views?
Would it be good to make something to go around the tube, to cover this
gap? Also, wouldn't this allow dust/spiders in during storage?

Or, is the gap to small to really worry about?

thanks