The 20 spin is much too big. A Capricorn spin would work if you found one of those or any other F18 spin.
Have you ever sailed a spin rigged cat? I would encourage you to go and sail with Rich on the Tiger or other well rigged spinnaker cat. This will give you an idea of how everything works and what you're looking for in a spinnaker. It's not like a monoull where you turn downwind with a big bloopy sail on the front and max-out your hull speed and that's that. The cut is important.
Have you ever sailed a H20? I would like to invite you to come sail with me sometime. The 20 is a powerful machine. The winds we get here in NorCal will put a 20 sailor to the test. It's a huge leap from the 18. The 20 is a great boat!
I would also encourage you to hook up with the local fleet (281, 240, 20) and get some tuning tips. Maybe do a race. We did a tuning seminar last year and a few guys that came out from Stockton that had been sailing their H16 for 20 years couldn't believe how fast one of our fleet's top racers made it go. He said he actually got scared from the speed of his own boat, which he bought at O'neills in the ,80's, for the first time in 20 years. He also said that he learned more from that day than he had in 20 years.
Even if it's your 18, bring it to one of our regattas this year. Sail with some other 18's. Learn the tuning. You'll learn more at one regatta than you do in 5 years of sailing alone.
Unfortunately, the 2 20's I was brokering have been sold, There will be more! Our division loves 20's!