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You should not discount PT plywood for transoms; but understanding the copper’s effect on aluminum, one needs to take precautions. My son made a PT transom for his boat, but he bought a sheet of 1/16" polyethylene to put between the PT and the aluminum of the stern, effectively isolating the dissimilar metals. And do not forget to put a dab of silicone putty on each screw to keep the metal of the screws from connecting copper to aluminum where they pass through the stern.
By the way, marine plywood is not the Be-All and End-All for transoms. Yes, it is much higher quality than construction or cabinet-grade plywood, with no voids, with (sometimes) plies from naturally rot-resistant tropical woods, but it is not treated to resist rot. And it’s not possible to put enough varnish on wood to completely exclude moisture. Wood finishes don’t work that way.
Perhaps I should post a paper I wrote for tinboats.net, about wood for transoms. It’s too long for a single post; Moderator, how do I do a link to it?
This from Wiki about Marine plywood:
MARINE PLYWOOD
Marine plywood is manufactured from durable face and core veneers, with few defects so it performs longer in both humid and wet conditions and resists delaminating and fungal attack. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. Each wood veneer will be from tropical hardwoods, have negligible core gap, limiting the chance of trapping water in the plywood and hence providing a solid and stable glue bond. It uses an exterior Water and Boil Proof (WBP) glue similar to most exterior plywoods.
Marine plywood can be graded as being compliant with BS 1088, which is a British Standard for marine plywood. There are few international standards for grading marine plywood and most of the standards are voluntary. Some marine plywood has a Lloyd’s of London stamp that certifies it to be BS 1088 compliant. Some plywood is also labeled based on the wood used to manufacture it. Examples of this are Okoumé or Meranti.