The USBC has already done all the work for you. If you check out their site, they have formats for every combination. Just select the number of weeks that you need. See link below for more information.
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League schedules can be found on the United States Bowling Congress' website at www.bowl.com. They have schedules for 4 to 48 teams.
Also, there is league software available on the market, such as CDE Software's Bowling League Secretary, that have the schedules built in.
In your situation, you would use an 18 team schedule. Then set a position round at week 8, 16, 24 and 32. When you insert a position round, the schedule gets pushed down.