I read about using aquarium tubing for treadle belts here some months ago and tried it. The tubing belt was to be wrapped around the wheels taking all the slack out, but not stretching it and cut where the end was. Then shortened by an inch or so and the cut off piece used for the coupler. To do that the short piece was slit length wise, then rolled tight and pushed into the two ends of the belt.
Here is what that looks like with my 9W-7 in the treadle:
Unfortunately that didn't work as good as I thought it should. After using the belt the coupler would slip out of the ends and the belt would loose it's tension and start slipping.
So I had an idea. Make a barbed coupler out of some of the acetel rod my wife bought for her crafts. I took a length of 3/16" rod and cut off a 1" piece. Then put it in my drill motor chamfered the ends and cut groves in it for barbs.
Here is a pic of the old slit tubing couple, the acetel rod and the new coupler in the tubing belt:
And here's a close up of the new barbed coupler in the tubing:
And here is the Singer 127 I'm using the tubing belt on right now:
I've mentioned this one before. It's the one in my thread: I never thought it would sew again.
The new barbed coupler has held for several days and almost an entire Whacky bag of sewing. I do believe it's a much better way to keep the belt together.