I got a different type of Slingshot (shown in the OP) Been shooting Wrist Rockets for 50+ years but decided to try something different. I just rebanded my two Wrist Rockets a few weeks ago with store bought band assemblies, and found that they are not as fast as my new Flat Band Sling Shot from Simple Shot. The new one is much easier to hit with as well.
When I was younger I used 3/8" @25 lb draw and 1/2" @ 25 lb draw Surgical Tubing for my Wrist Rockets and got much better performance from them. The store bought assemblies use a tube that isn't very fast and they will get new Surgical Tube or that Theraband material to replace the Red and Black (not Theraband) material on there now. Luckily I can still use the pouches with the new tubing.
I have also made band assemblies from Flat Theraband which is what my new one uses. The Maximum/ Optimum draw length for bands is 6 times the relaxed length. You work backwards from your draw length to the relaxed length. This is where you get the most power from your bands, however 5X gives you more life from a band set.
Just some info to help out.
Randy