I would loop a strap around each front fork and pull it down/forward at a 45ish degree angle to the deck, then use 2 other straps to pull down/back from the back of the ZTR to the deck.
Legally speaking you would probbaly be fine with a strap over both forks to tie the front down and another in the back, but (IMO) its worth spending a couple more minutes/dollars on 4 good straps (I would get ones with a WLL of at least 1000#). 2 more straps are much cheaper than damage to the mower (or injuries to someone else) if something goes wrong.
Aaron Z