I think a lot of that depends on what you're trying to do. I assume you're talking about a standard Hobie 18 rig, not an SX rig, and that you're looking for a custom asymmetrical spinnaker, not trying to match the dimensions of the spinnaker that was included with the Hobie 18SX. Do you have a spinnaker pole and a tang already set up on the mast, or are you looking for input on those as well?
I know that for a time Surf City Sailing sold a Hobie 18 spinnaker kit, and I believe some have retrofit their boat to use a Hobie Tiger spinnaker. There was a thread on here started a little over a year ago for folks looking to set up a spinnaker for the Hobie 18, using a spinnaker that Chip Buck with Whirlwind Sails had cut. I believe he was planning on making these to order. The thread has lots of great information on spinnaker setup, parts and equipment needed, etc. I'd highly recommend you check it out:
https://www.hobie.com/forums/viewtopic.php?f=13&t=57416