Been using these for the last 4 years, cheap, sturdy, sup leans against the wall horizontally (on its rail), I have one sup stored above the other, total of 4 racks, total cost $25.60 (plus plugs and heavy duty screws if you don't already have those).
www.bunnings.com.au/pinnacle-general-tool-holder_p2610012 See photo, my 8'7" is stored on the top racks, and my 10' lives behind the ping-pong table (takes 30 seconds to roll out of the way).
If you want your sup to lie flat on racks horizontally, then there are several videos available illustrating cheap ways to do it for a DIY-er.
