Actually, Wayne is correct. But there is a way to mount a 15lb bottle sideways w/o a hitch, as long as you tilt the siphon tube towards the back so it may siphon liquid, NOT gas. Keep the valve forward a small amount ahead of the bottom of the bottle. You'll be fine, experience says so!
Albert C. is an NX dealer from TX. I'm sure he could further assist you with the install.
Regards,
Doug