Thanks for all the suggestions. Just yesterday I was on a ride and was at a red light with a cyclist that had the exact same rack on his bike. He said he just squeezed the legs on the front wheel (quick release) and used an old tire tube to anchor the top of the rack to the top tube of his bike.

So, I went home and squeezed the legs onto my own front wheel (no spacers) and I ordered a special P-clamp (thanks Sch), which I'm gonna bend to fit when it comes. I'm using zip ties in the meantime.

Again, thanks for everyone's help and suggestions!