I am trying to model a block whose inside cavity is filled with a second material.  The block is a simple prism shape.  The inner cavity is essentiall a slot cut into it from the one end and goes about 3/4 of the way into it.  There are two small through holes at the other end of the inner cavity which exit through the opposite end.  The inner edges of the cavity have small fillets on them.

I tried to do a simple extrude of the same sketch I used to create the inner cavity but it cannot extrude up to the limit of the other end - seems as though the inner fillet causes it difficulty.  Is there a more simple way to fill up and inner cavity including the two exit holes?