I personally do not believe we can solve this puzzle completely. So much has happened in the archipelago since the birth of keris until this minute, and among the recorded either in tangible or intangible form, many are lost and forgotten.
However, further I think that at least we can contribute our ideas so that we have more options to think about rather than accepting the general conception as is. On this occasion I would like to describe further about the reasons why I recommend to remember Genderuwo and Banaspati.
We all aware that in the archipelago people do syncretism-aculturism in some degree, in this regard characters and stories that come from other countries, eg India sometimes are remembered in local names/dialects so does their character and genealogy may differ from the original. Outside the art of Wayang, many Hindu gods are long forgotten in everyday life.
After the entry of Islam in the archipelago, people change their idol. Gods of the old is replaced with a new one, but some spirit remained even to this day, like Genderuwo, Banaspati and Kala in Java. This apparently because in Islam the similar being are believed (Jin). These characters are already imbued to the local belief so that by mentioning above names for example, you wouldn't any longer directed to certain religion, but merely to folk culture.
If you want to depict Hindu Gods, people can describe it more accurately, I would very much agree with this. In my opinion, in an earlier era this is what mostly possible, the same as those still to be found in Bali, because they did not find any reason to replace it. Beyond Bali, generally, let's say we were in the late 15 C-early 16 C, adherents of Islam, of course we do not want to have our kris hilt depicting Bayu or Shiva. But people has live with keris since their early life, idolize it so much, inherete it from their beloved and respected forefathers, heard a lot of stories about how powerful it is, etc etc so well let's compromise, you can keep the blade but please change the hilt!
The stylized bird like figure such as found in Bugis, Sumatra and peninsular Malaysia style, might be acceptable. Probably something like Garuda into Burung or Ganesh into Gajah. We saw beak like feature in these bird-like creatures-although I generally regard them as nose because mostly we saw mouth-like feature beneath, stylized feather throughout the body. Sure it is much possible. But we shouldn't forget that creatures like what I suggest also told for being hairy in appearance. Although with doubt I personally thought that these style might be once trying to depict a more human like figure in terms of body structure, it's just stylized as far as possible so that what is actually portrayed really can not be recognized anymore, in this case is actually the purpose of the artist or initiatives behind this (Kyai/Ulama/Wali etc) is actually sort of works. In the next generation, this is probably even less understood by the younger who regard it as another character from what once described by a predecessor, or a mistaken understanding (like perhaps what we are doing right now) also has spawned a new form.
When we see far to the north to the border between Malaysia and Thailand, we can see a more human-like figure. This may be caused by contiguity with the Thai culture who still adhered to the belief of the old where sculpture art in a more naturalistic portrayal of figures can still be found today.
For the case of Cirebon and surrounding areas where this thread has been started, I am attaching a snippet from google book:
The Islamic traditions of Cirebon: cults and customs Among Javanese Muslims. In this excerpt we can observe that the figures I mentioned above is generally believed by the local community to this day. So it might be reasonable to speculate that one or some of these more generally accepted figures are what once described into what we now understand as Cirebon Giant hilts to replace a more specific Hindu Gods/character.
Thank you.