I like AME's suggestion of getting something that matches the shank of your Ering. Therefore, a full eternity ring (shared prong) with melee size close to that of your shank will be great. To avoid potential problem of diamonds from adjacent ring eating into the metal part or damaging the diamonds of your Ering, you may want to sandwich the rings with a thin spacer (1-2 mm wide, depending on the width of your shank and your taste). Another option, that eliminates the need for a spacer, is to get a channel-set eternity ring (similar to Whiteflash's Honey ring).
As much as I prefer the clean look of a single, solitaire diamond ring (to let the center diamond stand out without distraction), I always pair my solitaire ring with a 2-mm plain band (which I also use as a spacer ring when stacking) or a channel-set "Honey" ring. Reason is that without a second band, my E-ring tends to spin with the top-heavy diamond landing somewhere to my middle finger or baby finger.
As for LIllyan25's suggestion about getting a custom made curved band, I guess the motivation is to avoid a gap between your Ering and the stacked band. If the design of your Ering is such that the cup holding the pear diamond is higher than the stacked band, then the gap will be minimized and you will not need a curved band. When selecting my solitaire setting and stacked rings, I take the rings' height into consideration.