Any way to do a mitered corner without a table saw?
Your circular saw should be able to cut the 45 degree angle, there should be a lever to allow you to tilt the plate thus tilting the bade.   Something like this


Then to get a Straight cut you need a guide along one edge. You need something longer than the piece you are cutting and wide enough to clamp it to it and have enough room to run the saw along the edge for a straight cut.  Something like this, although this is a special clamp guide, you could just use something a bit wider with some other clamps.