Well looks like I'm a little late to give an answer on this thread from my own experience - but figured I'd post anyway for future reference. I've had MM cut me several 20ga. solid slug molds as your describing. The internal bore diameter of various rifled barrel slug guns varies from 0.615'' to 0.625'' with the larger diameter range being more common. Slug your bore or at least measure it about an inch or so down into the muzzle with a pin type set gauge and then calipers or a micro-meter to check the pin gauge.
Long story short full-bore works great even with the forcing cone issue. Here is a picture of the general shape I made my mold:
And here is a link to a thread where I shared some of the slugs from the mold with another guy and he did independent accuracy testing on them out of his gun -- picture of a nice tight group included on the second page, third post down !!!!