Ah. The stem is a convenience at best and a useless tradition at worst when building with plywood. You don't need a stem even for plywood. You can just stitch it all together.
For all Plascore I don't make a stem at all. I just stitch the Plascore together and call it good. I do add a block of wood, whittled to shape, to go where I will put the trailer eye bolt. Many builders put that trailer eye bolt half way up the stem area, so the rope from the trailer winch goes straight to the eye bolt, parallel to the ground.
I put the eye bolt down at the bottom of the stem area so the boat jumps up onto the trailer roller without anybody lifting anything. I want loading the trailer to be a one man job. Without busting a gut. Others complain my way is no good because the trailer winch strap now points down (once loaded) which allows the front end of the boat to bounce on the trailer as you wobble down the highway.
It's never bounced for me. The strap that goes through the oar locks holds the boat down tight as you please. If you are worried about it you can re-route the winch strap after loading, so the strap goes under the trailer tongue and then attaches to the boat. I don't bother. Bouncing never happens. Not if the boat is strapped down with a good winch strap going through the oarlocks.
I always always put the trailer eye bolt at the bottom of the stem area. The boat jumps right up on the trailer and all I have to do is wind the winch. It is interesting how few others do it that way, including most factory made boats. And how many guys you'll see at the boat ramp, groaning and busting a gut as they lift the front end of the boat up onto the tailer roller, while the other guy barks commands and winds on the winch.