We went with
these last year, and turned down what I believe were 4" plactions to replace the steel hubs they come with. It was a shoddy solution, because we ended up turning them down so much that we took off the entire rim, such that only the spokes contacted the inside of the wheel, which destroyed some inner tubes during competition. It did hold up though, although I would much rather use a different solution for the hub if we ever used pneumatic wheels again.