Or youd need a dielectric slab to insert into the C band feedhorn at a 45 degree angle to the 2 probes. I believe the DMS international ASC421 LNBF is made to accept a dielectric slab and has little ridges along the inside of the feedhorn to support the dielectric slab and hold it in place. In theory you could do this with any feedhorn or LNBF as long as you have a way to secure the slab in the correct position. Of course doing this will degrade your reception of linear polarity signals. I have seen pictures where some people have placed the slab just outside the mouth of the feedhorn by cluing it to the plastic cap that covers the opening. This way they could easily remove it by removing the cap when they want to go back to linear polarity.
EDIT: here's a link to a page with a picture to show how the slab can be attached to the feedhorn cap.
