From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger permits a woman to use a moch after intercourse but only in the manner of a standard bedikah of the outer chamber, relying on the Rosh and Rashba (as codified in Shulchan Aruch) that such preparations for counting seven clean days are permitted. He notes, however, that this shallow examination likely has no actual contraceptive effect, since viable seed lies in the inner chamber beyond the moch's reach, and deeper forceful insertion would likely be prohibited.
Source: Shu"t Rabbi Akiva Eiger §72