There are plenty ways to make up this air gap. Some ways are better than others. Others will tell you to wrap string around the blank and soak it in epoxy. Both of these methods will work, but neither one provides the best option for transmitting vibration or staying together year after year, through cold, and heat and wet and dry and all the beating that we all put our rods through. 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.