You do not need a "keyway" on plywood. Yellow glue alone is enough to
hold the trim. As others suggested you can brad nail or pin nail the
trim until glue dries. If you do not want nails, you can glue up with
cauls.Bore holes in scrap 2x material the same diameter as the half
round.If you forstner bits use them, they cut cleaner.Rip in half, now
you have half round caul with a square edge for clamps. Do all
finishing on plywood first before glueup of trim. This way you can
clean any glue spillage with a damp sponge. I'm assuming you will put
on a clear top coat.
Temporarily tape the glued trim to the plywood ,tape one caul to
plywood , add caul on opposite side and clamp up. Figure on more clamps
and cauls than you may need.I would imagine 6" centers at most.Make
sure the top edge is even or slightly proud of the plywood. If top edge
is a hair above you won't see it, if the plywood edge shows at all, it
stands out like a sore thumb.