Do not tether it !!!!!!
Tethering it will cause more problems than its solves.
I own and fly fullsize but have flown dozens of my own model helicopters in the early years when power/stability were big problems.
A good idea for first flights would be to simply increase the width of the undercarriage [sideways and for/aft] with lightweight beams to prevent rollover. pref with rollers at extremeties.
Good luck. Dont like the idea of belt drive to tail however. Never good on models. Shaft drive only way for power and reliability.
OOW