[Andy A.]

We used sail insignia cloth, cut by hand with a razor using a stencil. It's durable and self adhesive; we haven't had any issues with it coming off or wearing out.

The fabric is a specialty kind of thing, so it's not the cheapest option. It'll go for about $20 a yard online (each yard is 54" wide, should be plenty for a set of bumpers). You might find something functionally equivalent at a Joanns or the like for a little less.