There are, literally, hundreds of material choices you can make to build your boat, Dave. Some will probably chart-out as better choices than others depending on what it is you'd like to do with your boat and where you'd like to do it.

I see that the same old comments about foam as a viable core for a kayak are still floating about on the forum, with the same limited understanding of how the material would be used. It's more than amusing to read, but it is functionally incorrect.

A foam cored boat is not always the best choice for any boat to be built, much less a kayak. It is really, really good for certain types of applications and not so good for others. You really do need to arrange the priorities of the uses intended in order to make a sound choice for the core material.

Your query about using some type of plastic is workable if it allows you to achieve the shapes you seek, the workability you can handle with your skills and tools and it gives you the durability needed for the type of paddling you desire. None of them are any better than others, as it is a purely personal decision that will come from your style of boating. Wood is a good material, but it is not the only material out there from which to build a boat.

I encourage you to explore the possibilities.