We had some 5/8" ply left over from putting up a shed, so the memsahib asked me to cut the largest circles I could get out of them. I figured the easiest way to do this would be with the cheap (29.95) spinsaw I bought at Canadian Tire last year, but the trammel will only do a 12" circle, and I needed 22½".
So, I got to work, went down to the local HD and bought a narrow aluminum metre stick and a couple of flat head ¼-20 screws and nuts. Then I cut the ruler off at 18½" so I could cut a 36" circle (or 90cm. if necessary)
Then I drilled a couple of ¼" holes to match the existing trammel, which connect to the base of the spinsaw, countersunk the holes,
and put the whole thing together, drilled a hole big enough for a #8 screw, and put it into use.
It works excellently, is much lighter than my router, and I can use it over and over again. I might look for a ¼" blade, as I broke two 1/8" ones cutting out four disks.