As you have mentioned, High Density Polyethylene HDPE) is relatively inexpensive with a 4'X 8'X 1/2" sheet costing about $125.00. That's more than enough to make all cross sections, coaming, backrest, etc. for 2 kayaks. In addition, it is extremely durable.

One of Polyethylene's best features is the ease with which it can be cut and drilled. On an SOF folder, holes can be drilled and then cut slightly beyond the halfway point to allow aluminum stringers to be "snapped" into place. This eliminates the need for complicated brackets. This should work equally well for non-folder use.

Polyethylene is a bit heavier than plywood, but due to it's inherent strength, can be drilled with lightening holes, or thinner cross sections can be used to reduce weight.

Unlike wood, it requires little or no sanding, and no finishing. When used in conjunction with aluminum stringers, there is virtually no finishing work required to make a kayak. Time better spent on the water.