Other than blown rotted Valleys, my leaks have been rim to kayak leaks. Like Andy, I have a Kajaksport kayak (mine's a Viviane), and it's got a large oval on the front, and an extra large oval on the back, and they don't leak. P&H seems to have had issues sealing the rims to their kayaks, at least for a time a number of years ago anyway. I resealed a couple of mine and one of my friends, I believe all of the '03 to '06 years. I think the round ones are a little less ugly on the deck, but I think function outweighs my personal opinion about looks.
If I were building one, I would certainly note a couple things. Leaks between the rim and kayak have shown themself to be common in my experience. All these tupperwear lid covers can be a bear to put on and take off in some situations, so you commonly see a lot of pressure put on them, including things like smacking them with the paddle as noted above. So the intallation of the rim has to be very, very solid.