That's where arbors come in. Arbors are small, round cork or graphite pieces that are glued to the blank. The reel seat is then glued to the arbors to complete the installation. When you order your reel seat, just make sure you order arbors with the same O.D. as the reel seats I. D. Cork Cork is the easier of the two materials to work with if you are building your own arbors since you can turn them right into the cork as you are gluing up the handle if you are building from scratch. Cork arbors can usually be purchased turned to the correct outside diameter when you buy your reel seat however, so there should be no need to worry about buying extra cork rings if you elect to use it for arbor material.
Cork is lightweight and strong.
The only disadvantage is that it is a natural substance, so you may run into internal flaws as you are fitting the arbors to the blank that cause you problems as you try to get everything assembled. Even though cork is a very durable substance, it can also rot, which can leave you with a reel seat that twists around the blank when you try to use it. This can be fixed without tearing the rod apart to repair it, but it does take time and it doesn't leave the seat assembly looking like it did when you started the repair. These arbors are made out of lower density graphite that is easy to file to fit it to different blank diameters. The nice thing about these arbors is that they are man made, which means that they are more consistent throughout and there should be no surprises as you file them to size.