John wrote:
I need to cut a few 1/4" panels to cover a box so that no joints are
visible.

If you're speaking literally, you may be setting your expectations too
high. I'm not exactly sure what you mean by panel, but if you're
talking about sheet goods such as plywood you'll have a tough time
making those miters perfect. Repeatability and maintaining perfectly
square pieces is tough enough with a table saw. Laying one box over
another is even tougher.

There are 4 ways to make miter cuts with a circular saw:

(1) clamp the keep side and saw with the motor on the keep side
(2) clamp the keep side and saw with the motor on the throw-away (drop) side
(3) clamp the throw-away side and saw with the motor on the keep (drop) side
(4) clamp the throw-away side and saw with the motor on the throw-away side

(5) Tell a cabinet shop or woodowrking friend, "I'll give you this
circular saw if you'll cut these pieces for me."

For simplicity, I'm only interested in choosing between (1) or (2). I
experimented with these two and the result was inconclusive. (2) seems to
work slightly better but could also be because I gained some practice after
doing (1).

You don't want the saw to move in planar relation to the piece being
cut. I find keeping the clamp and saw on the workpiece the most
accurate.

What I learned from the tests is I need to keep the saw absolutely level
because slight change in the elevation would cause the cut to become
crooked. I wonder if there is some sort of saw guide to help keep the saw on
the panel; it is too easy to tilt the saw slightly.

If you have the typical sidewinder circular saw with the motor to the
left of the blade, most of the weight of the saw is over the wider part
of the base. Just concentrate on keeping the saw base against the
fence and pressed down against the panel. Start the saw a bit before
you hit the workpiece and cut all the way through on the exit.
Sometimes I cheat and let the saw wind down inside the kerf as soon as
the cut is completed, but that sometimes leaves saw marks.

R