I ended up with a simple slot, 11.25" long, 0.75" wide, with a 4.5" shelf. Not perfect, a resonance or two, but it sounds musical, well balanced with impact, and it goes deep. I can live with it for a loooong time this way.

Probably the most fun was working on it with my 4-year-old daughter. You would think she would play with the mountain of toys in her room, but no, she decided daddy's lengths of 4" PVC sewer pipe were more fun. With the different lengths, we had daddy sewer pipe, mommy sewer pipe, baby sewer pipe, grandma, grandpa..... sewer pipe dress-up, sewer pipe tea party, and finally sewer pipe Bozo Buckets.

Thanks for the help, RBP. I'll give it a go again this summer.