Milled fiberglass will add shear strength. Colloidal silica will make it thixotropic (a fancy word for syrup-like so it stays where it is put). Wood dust (aka wood flour) and microballons will also thicken and lighten it - for used as a filler or fillet. And also usually cause the epoxy to "kick" faster. Mix the epoxy plain first and then mix in the fillers before applying.

Gererally, anything you add should be for a specific reason. As a cement epoxy doesn't need anything. A better idea for wing halves is to bridge the seam with 1 oz. fiberglass cloth.

PS - don't clamp epoxy tightly as in white and yellow glues. You want a thickness between the parts, not mushed out. That's called a "starved" joint.