The simplest way to stretch Rubber Tubes I know, just add simple Plastic Wall Plugs, and some kind of thread!
Works with the plug's screws, too, of course.
Can be used for Slingshots, or any other Project using Rubber Tubes, as spring substitutes. Might also be good for Plastic and Silicone Tubes, for other uses, if there's a tight fit.
While some may just look at the first Photo and get the idea, others may not, so I've elaborated on the subject. Feel free to skip any Steps, and get to do it, if you do get it.

So here's what you need:

-Wall Plugs, 2 per Rubber;
-Rubber Tubes, of course;
-1mm Nylon rope, or other strong chords;
-And an X-acto knife, or as in this case, any Pocket Knife

Step 1: Cut the Plug's Tip Apart

Makes things easier, just open the thing with the knife, knife tip AWAY from you.
If you're planning to use a Screw, however, skip to Step 4

Step 2: Open Up Your Mouth, and Say "aaahhh"

Here you see the plug's tip open, and the rope already in, so after that, close the tip.

Step 3: Ready, and Set

See, it's easy, but remember to set the rope, in the middle of the Plug, bot potions sticking out, equal in length.

Step 4: Plug the Plug

Now, what's left to do, is to plug that Plug into the Rubber tube, be sure to chose a Plug that it's going to be a tight fit inside the tub, not a loose fit...