#8 Screw
A #8 screw is 0.164 OD. An AN426-5 rivet is 0.156 OD and an AN426-6 rivet is 0.187. The -5 is probably best. If you are just filling holes try to get an AN426A5 rivet, the A material is 1100 aluminum and will be easier to drive, but with reduced strength compared to an AN426AD5.
If you are just plugging holes in thin sheets with no structure behind it, be careful not to deform the sheet. Try practicing on some scrap before plugging the actual holes.