I built Nick's stitch and glue kayak out of 1/8" luan plywood which was carefully selected from my local do-it-yourself warehouse. I cut the panels and only found a significant void in one of the panels, which I simply recut. The kayak was glassed with 6oz fiberglass inside and out with doubling in a few high wear areas. I also put in 2 bulkheads of the same material. The kayak is very strong (we accidentally dropped it when lowering from my third story apartment. It fell 8-9 feet on to the bow and we can find no damage!). If I was to build another stitch-and-glue, this is the route I would go. The 1/4" plywood seems to me would be harder to bend to shape but would (I assume) be even stronger than my 1/8" boat. Whatever material you choose, good luck!