There is no problem using a stent. Some people get queasy cutting a blank but if you are careful, it isn't a problem. You have to be careful not to crush the blank, but it doesn't have to be neat. It's going to be hidden under cork and a reel seat.
If you have a large diameter blank (meaning non-graphite), you can get an insert that is sized correctly. If you are ordering from Belenger or REC, the inserts they come in different diameters (see
www.realseats.com). If you are willing to eat the shipping costs, I expect you could send your insert back and get a larger bore (or just buy one for a larger bore). Given the other costs in the rod, that might be justified.
Ron at Southwest custom tackle sells Lemke seats and will turn an insert for you for any seat. Ron also will warn you when the insert is too thin and you should use a stent. I'm sure there are other people around who would turn a seat for you, but Ron has a lot of experience at it.