Nugget, I wouldn,t go to a chest mount or bungy cord system i would use a swing arm.
https://www.prospectingaustralia.com/forum/img/member-images/1615/1406328337_swing_arm.jpeg
I use this with my GPX5000 this gives you options 1- lay the coil completely onto the ground and skid the coil over ground only lifting the coil to check ground balance. 2- hover just above the ground as normal. 3- remove arm from detector grasp detector by cuff strap and slide coil under bushes, logs etc. This system gives you 2 meters of reach with out moving your feet, very handy.
Bungy cords are a pain in the A** as far as i am concerned they are either to high or to low etc.
Unless you plan to go into water this obtion will be ideal for you and it gives you more options and uses over the chest mount.
Bolt one of these to your tdi and you will be amazed how the second arm helps with the weight of the unit and comfort.