I am building one of the DeltaII rockets offered by Phillip Landesburg(sp) and am having a heck of a time cutting out circles for formers inside the body tube. I found some paper tubes to use as forms for the SRB (tampon applicators) that were just the right size, but didn't have anything handy for the body. I find that even with a circle cutter, my circles are varying significantly in diameter. I don't think the adjustment is slipping. Any ideas?