I'm not sure what stores you have available within reasonable distance but there are some very bright LED lights available these days in various forms and easily portable.
One thing I would try is to light it from one side and then the other (down the length of the mound) so that the face of it is illuminated but the indented areas remain in shadow, that may reveal some shapes/lettering to all of us that may be hard to make out otherwise.
I'm gonna take your images into photoshop and play around with them later if I get some time, to see what I can come up with.
Either way this has been a very interesting read for me and a memorable adventure for you and others I'm sure
HH,
Chris