Looks good Moni.
Nice thing about the Warhammer Chaos stuff is that the vehicles have been warped and changed to a greater or lesser extent by the influence of Chaos. So all sorts of wacky modifications can be acceptable, fungal encrustations, tentacles, screaming faces embedded in the armour etc.
Assemble the 'rubber band' tracks in the usual way and place them on the running gear. Then get something to squeeze between the track and the skirts in between the return rollers to create the sag you require (rolled up paper will do the trick). It's then just a case of putting a little superglue on each side of the return rollers (where the track leaves the roller). Leave the rolled up paper (or what ever you've used) in place whilst the glue sets. Once set, remove the paper and the track will have the sag you want.
Hope this helps