I'd consider clustering by only customer_id, unless the selects themselves order by both customer_id and thing_id.
I'd also use a lower fillfactor for these indexes, maybe starting somewhere between 80% and 90%. A few page splits aren't that bad, but you probably don't want every page immediately splitting.
SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".