I installed a 5 gal. aluminum racing tank from Summit. It has an outside fill and uses a Facet pump to transfer into the nose tank. C.G. does not appear to be a problem as long as the nose tank is reasonably full when there's fuel in the auxiliary tank.
I also have a Onex with a custom, 6 gal, wedge shape tank that conforms to the seat back. This tank has an inside fill. Once again, C. G. is not a problem. However, gross weight is and I can't carry much else or eat breakfast when there's fuel in the auxiliary tank.
Vince Homer
Onex, Jab 3300, N362DV
Sonex, Jab 3300, N362DR