Tape tip!

This is probably what everyone else is doing already but I dont think I have read of this cheat before. So just in case this is a novel idea here goes.
Trying to produce small circular masks (from Tamiya tape) for the dorsal windows of the Lancaster and several abortive attempts of both cutting in situ with a blade or after transfer to my cutting mat I decided to transfer the pencil marked tape to thin paper and cut with a pair of dissection scissors. This produced a much more acceptable "round" mask and was easy to separate from the paper without damage to the mask itself. Hope this helps someone!