Rush could do it. I don't know of any other conservative presstitutes that could be even near reasonably objective.
A Savage interview would be entertaining, but candidates could decline it without losing face.
Candidates could not easily decline a debate cosponsored by market leaders Rush and Hannity, who are both appear to be fair questioners. If you combine Prager with one of the market leaders, given his pure reputation for fairness and honesty, the debate would be hard to decline.