Update: I bought the URC 8820 from Amazon for $14. It has a similar key layout as the Cox remote because they are made by the same company. However, it lacks a few Cox-specific buttons -- namely "Page+" "Page-" "A" "B" and "C". So I programmed "|<<" and ">>|" on the URC 8820 for Page+ and Page-, then the lowest left 3 keys as "A" "B" and "C". (The URC 8820 learned these keys from the Cox remote.)
The URC 8820 has codes for Onkyo receivers, but none is fully compatible with my 604. Some codes turn on the receiver but not turn it off. Others would turn off but not turn on. Weirdest thing. I solved it by picking a code (1298) that works with the 604's volume functions, then re-programmed the "Power" button from Onkyo 604's remote.
Now I have a remote that looks similar to the Cox remote and works with my Onkyo (and other devices, DVD, TV, etc). For just $14!!! I'm happy. :-)