I would go ahead and buy a wheel chock from Harbor Freight [like
this one] Bolt it down to some 1/2 or 3/4 plywood to give it a strong base. This will help keep the bike from swaying and from there just strap it forward and down to keep it from moving too much. To keep the least amount of movement, put the bike towards the front.
Another thing I have heard some people do is use a mattress in between the bike and one of the walls and strap it that way. If you do this, you may want to consider running a ratchet tie down from wall to wall to relieve some of the deflection on the wall.
Also, some box-truck rental places [at least U-haul] don't allow you to rent a truck if they know you're going to put a motorcycle in it, so bear that in mind when you go to rent it!