I am sure this should be realy obvious. I have a 4 inch wide 6 inch long secondary mirror for a 20 inch primary. I want to make the secondary holder out of aluminium and then stick a carbon fibre sheet to it and the secondary to that. I am going to cut a round disc of alumium and then cut one end at 45 degrees. What diameter should I cut the disc to? 6 inches? Should I make the secondary holder a bit smaller than the secondary? I have a six inch hole saw I could use to cut the disc, so I will probably use this if it is the right size.