I would suggest borrowing a canister light. That should be allowable. . . .
That's what I did for a Cavern course. I simply told my instructor I needed to borrow or rent a primary light, and he said no problem and lent me one. Since a cavern is, by definition, in the daylight zone, having one for a Cavern course is more about getting accustomed to holding (Goodman handle) and using a light for signaling while juggling it with the reel, etc., than needing something very bright and/or with a long burn time. After that, I bought a used Light Monkey 21W HID--those old workhorses come up for sale now and then on ebay, SB, CDF, etc.