Is it ok to build with an 8" sonotube?? Why not. Gives you more room to properly baffle the tube so stray light won't get into the eyepiece. Baffling is done by drawing a line from the edge of the top of the tube to the mirror edge and calling that the clearance line. The next thing is to put a baffle just above the secondary mirror so that the eyepiece sees black black surface and then take a line from the far side of the end of the tube and draw a line and where it hits the tube is where the next baffle is needed. You may also start by defining how much of the end of the tube is needed to produce a 100% illumination of the primary at a particular angle, say 1 deg. offset from the centerline. but in any case, have fun and enjoy the stars.