I found that I needed more in-line supports than I see in the photos above, Your arrow is gonna start vibrating and moving as it spins if you do not have support in several spots. If you can devise a fold over roller that surrounds the shaft and keeps it absolutely from bouncing, that could be a cool addition.
Keep the tubing section short and even try inverting some of the tubing over itself to double its thickness (thick end on arrow). If it is long, it will oscillate (?) and make the arrow bounce. The object is to keep the arrow spinning as straight as possible for getting nice straight lines.
Maybe glue in a support block just under the cresting area that comes up to within 1/2" of the shaft. If this extends out toward you, you can rest your hand on it while cresting and you can also tape a stencil on it so you can make all your arrows the same.
I don't like the foot driven rheostat on mine (my hardware is similar to yours). They seem to bump up the speed really quickly (it is too slow, too slow, WHIZ ! ). Faster is not better.
ChuckC