Substituting sugar with a liquid stevia concentrate in a cake
Use dextrin.

On the theory that dextrin (insoluble fiber) is molecularly similar to sucrose, I made chocolate chip cookies and substituted dextrin 1:1 for all the sugar, sweetening the dough with erythritol (and regular chocolate chips).  I used the CVS brand dextrin which is sold with fiber supplements like Metamucil.  A brand name for dextrin is Benefiber.  It is very much a bulking agent exactly as you request.  It has no flavor.

The dough was the right consistency and the end product was more the consistency of shortbread than a toll house cookie, but they were good.  No GI issues either.