I would use a frequency generator type device. Not much use if you can't use L-rods. Some would say still no use.
Something else might try is one of those IR cameras to find the opening. There is an inexpensive one that attaches to a smart phone. I paid $200 but don't know what they have now but certainly better than what I got. Telephoto lens etc. SEEK Thermal (Thermal.com) is the name and I'm not sure what countries they sell to . Those expensive ones Kellyco sells cost thousands and they are not even a camera.
BTW, a cheap smartphone will run much slower, a time lag of a couple seconds but it still works. I don't know if this will work for you if the entrance is deeply buried.