Though I no longer have either, several times while on my Husky walking/slinging adventures I forgot to bring a sling.      
    On one occasion we walked to a quarry lake that was surrounded by yellow police crime scene tape because there had been a recent drowning there.  I made an Appu Sling from the tape.  It slung some egg sized stones for ten or fifteen minutes before it stretched too much to be of use.
     Another time when I had forgotten a sling we walked past a pile of trash.  Included in the trash was an old TV.  I pulled the electric cord from the back of the TV, split it, and made another Appu Sling from it.  This sling was durable and worked surprisingly well. 
     I have made many other slings of opportunity from  pieces of rope or twine I've found, pieces of cloth, trash bags, etc..
    Good post. 
      

Here’s a little sling preview for the trash sling challenge. I am editing the slinging video that goes with it tonight. Obviously I can’t win my own competition, but I wanted to give others a little inspiration

Just like everyone else, I don't have a video yet.  That should happen tomorrow.  But here's my sling and how it came to be.

The retention and release cords are made from the leg seams of a pair of blue jeans.  I dug out an old pair of jeans with way too many holes to be of further use.  The loop and pouch connections were put together with contact cement and reinforced with some extra pieces of hemp twine.  On a side note, I also got some material from the legs themselves that I'm going to turn in to sling pouches.

The pouch is made from two things.  The rings are plastic spacers that were originally used to separate rare earth magnets I bought a while ago.  I've been keeping them around trying to think of something to do with them.  The plastic rings are then tied together with some little pieces of hemp cordage.  That cordage was just excess which was cut off the end when I got done braiding a sling.  The pouch is kind of like a plastic and hemp "chainmail".

Hopefully I'll be able to get this thing out tomorrow for a test.

Leg seams! That's genius. I'll remember that one.

This is my submission.
I guess it's not overly pretty and I could probally throw as far but hey it's trash and it's a sling.

EDIT: just realised I didn't follow the instructions , however it is very capable of throwing over 5 times , the videos are throws that are somewhat accurate, I can upload more if I need to provide more evidence but the throws are pathetic ha ha
Second fail is its only made from one material , a plastic bottle , I missed that I should combine materials but I did use a flame(stove top) to melt and stretch the "cords" ha ha.

Nice Mersa, that's a great idea!  Great videos so far guys!

Here's my entry.  I was also outside the rules slightly.  I don't actually have a tennis ball.  I'm using my street hockey balls which are of a similar size and weight.

https://youtu.be/WPxiGgaY77Q

Thanks man!  I liked the sling, but it didn't work anywhere near as well as I was hoping.  Whose idea was it anyway to make slings out of junk.
Definitely a fun experiment!

And on the SITH sling, I got it some time ago.  I'm pretty sure I had the first sling picture in the SITH topic.  If you missed your 'Thanks', open up page 6 and scroll down through.

I've already had it out, but the unfortunately the ammunition I was trying to use was a little too large (was not an ice ball ).  I've got an another idea and am looking forward to the next round with it.

Ok... here’s where we stand:
Kick made a sling, but hasn’t posted a throwing video yet (perhaps a little distracted by Mallorca )
Mersa made an excellent trash sling, but only used one material. Video evidence of the sling surviving multiple throws was good.
Joe Meadmaker used multiple materials in his construction and threw rocks instead of tennis balls. Since the point of tennis balls was to prove the sling could survive several throws, I am ok with replacing balls with rocks.

So far, Joe is the one to beat. I’ll post a photo of the prize sling soon in case anybody wants to sneak in and try to steal it from Joe