Would a hardware or plumbing store have a blank 1/2" plastic disk I could
drill myself? 1.5 gpm is still too much.

http://www.pexsupply.com/Jones-Steph...rictor-2-5-GPM







How about a nylon washer ??

http://www.homedepot.com/p/Unbranded...7518/202210267

Looks close. But that's a big orifice. Maybe 10 gpm.