Taping or rubber banding to the boom is fine to see what your hover speed is. Thunder Tiger even makes a tach expressly for this purpose.

Your kind of limited though.

Holding your tach over the rotor is definitely a bad idea.

The Min Air Skytach is the best way to go, but a governor works well too. If you can only afford one buy the governor. You can always borrow a tach at the field.

Blair