For gluing rubber - in aerospace, we used instant-bond Superglue to build big o-rings - use a fresh cut on both surfaces to be stuck together, as a cut rubber surface seems to build an oxide layer after a short time. This degrades bond strength. Other glues will require some sort of clamp or fixture.
Superglue doesn't seem to spread well after putting the parts together, so coat 100% of the contact area with a thin layer of adhesive before bonding. Not only will the glued jint be stronger, there will be no little openings to leak rainwater.