Steve that is not overkill. One trick to save wight would be to radius in the side points to spread the stress over a greater area. Lightning holes can also be added to the plates. The plate must be strong enough to handle the loads it will cary but 1/4' ply is pretty strong. I think the main thing is to have it anchored well to adjoing structure. And that is done in wood by increasing the joint area. Also even if the joint is strong enough not to fail, you can still have a failure. The area the stress impacts must also be suffeceitnt to absorb the loads.
So spread it out baby, spread it out. From Ken engineering syllabus 101)
Look at how I tried to distribute the strut loads on a relatively weak wing structure on my CL RingMaster bi plane. Not quite what you need but the ideal is there.
Ken