Making a circle

and cutting it out sound simple doesn't it?
I need to block a knitted cotton hat. For the non-knitters among you this involves pulling - sometimes gently, sometimes vigorously - your knitting into the desired shape. It also has the effect of ironing it. It is an essential part of finishing your knitting.
Most things can be blocked flat. I have a number of polystyrene sheets (scrounged from a white goods shop) for this purpose. I also have "blocking wires" and "t-pins. I would really like one of those "woolly boards" for garments but they are expensive to buy and making one is now beyond the Senior Cat. I should have asked him at least ten years ago.
These things though do not work for hats. A hat is an entirely different shape. I have blocked other cotton hats over the top of a straw hat or a plastic form that I rescued from a shop which was throwing it out. (G..., who gave it to me, was amazed I wanted what she thought of as rubbish but the Senior Cat covered it in a little papier mache and it has served me well. 
The problem is that both these things have been slightly larger than I actually wanted and there is no way of changing the size. I need something smaller.
Hat "forms" or "blocks" are extraordinarily expensive. I could not justify the expense. 
I went on line and looked at possibilities. All of them would still need me to do something about the brim. Hmmm... I bought a "head" in the right size. I worked out how I could put the  head in a circle of cardboard and work from there.
And that is where we have run into strife. Middle Cat cut out a circle but it proved a teeny bit too small. The Senior Cat asked me to order a Stanley knife "because I want it for other things as well". (I didn't know he was going to cut carpet but...). 
I ordered said knife. It has come. The Senior Cat and I investigated. We finally worked out how to put the blade in - but the blade has jammed. He cannot cut the extra half centimetre from around the edge of the circle. The cardboard is much too thick to do it that precisely with scissors.
When he has worked out how to release the blade and get it to work he might be able to do the job.  Then I will  block and stiffen the hat  - at least I hope I will.
In the meantime I am thinking it would be easier to knit woolly hats without brims. They still need to be blocked but the same problems do not present themselves. 
And really, all I need is a circle cut to the right size.... sigh.