Have embedded several ways.

the megacassette can be put on the back of your sample, so the cassette
sticks up enough (part of the cassette usually clamped in the microtome)

Be sure to have the cassette above the sample, this may require an empty
cassette spacer below, then add the megacassette, making sure there are no
bubbles, and that the megacassette is filled with paraffin, to make
it more stable.

The old style embedding ring can be applied in the same way, as long as
you can clamp it into the microtome.  Just make sure you have
enough of the plastic portion of cassette available to clamp.  A practice
run maybe?

With that large a bone, I would also process longer, and if anything
else, do an extra long infiltration with paraffin in the end.

You did not say what kind of microtome you have?  Make sure you have
the proper clearance for cutting past the knife.

I recall an article on this in the Shandon LabLeader newsletter, you
could call them and ask for a copy, it had photos to show how the
embedding ring? or cassette was put on back of a large block.

Gayle Callis