I want to add a few additional shelves into my kitchen cabinets:
3 short shelves: 350x280x16
3 longer shelves: 735x280x16
I measured up 3 of the 1200x300x16 boards https://www.bunnings.com.au/flexi-storage-home-solutions-1200-x-16-x-300mm-white-matte-shelf_p2710141 and seemingly good to go.
I picked up a Ryobi 1500 corded circular saw https://www.bunnings.com.au/ryobi-1500w-184mm-corded-circular-saw_p0013549 along with a 60T saw blade https://www.bunnings.com.au/diablo-184mm-60t-ultra-finish-circular-saw-blade_p6370898 to make the cuts, but I've run into two problems.
The first problem is that the Ryobi has a 16mm arbor size, but the only 60T saw blades I can find have an arbor size of 20mm. The blade comes with a 16mm reducer, but the flange on the Ryobi is only 16mm and won't hold the blade securely. The hex nut has a small outer flange attached to it, and I assume that's what is supposed to go on the outside. There's also a larger inner flange that will cover the arbor, but I assume that must stay on the inside. Can I replace the outer flange with something wider to fit a larger arbor size?

This brings me to the second problem. I went ahead and used the 24T blade that comes with the saw on one board to see how it would go. First cut was fine (but messy, due to roughness of the 24T). However, when I tried to cut 300mm down to 285mm I found it really difficult to seat the saw so that it would cut straight and not wobble. I had the short side pressed up against a straight edge (a long level clamped to a saw horse), but the long side of the saw hanging off the side. I couldn't figure out the best way to setup so that the saw could sit flat. Any recommendations for cutting at the very edge of the board?
First time using a circular saw here, and strongly considering just using a hand saw (which I did use to finish the cut, and it was remarkably cleaner than my hack job using the circular saw.