Tie a couple of knots in some shock cord (also commonly called bungee cord) and stuff the knotted end into the slot in the side of this thing. Then you have a little plug insert that will retain the cord inside the end of an aluminum pole like the ones commonly used on camping tents.
Since it's likely this would be used on tents, chairs, etc outdoors in the heat, or might be kept in a car, it would probably be best to print in ABS or something that can withstand higher temperatures.
This seemed to print best with the end-cap part on the print bed. I tried printing it sideways too, because I thought that might make it less likely to separate on layers, but the print was a little sloppy that way with the support material. Would be curious if anyone finds a way to print that's both neat and strong.
This should print roughly the size needed for a pole with an inside diameter of around 10mm. Scale it up for larger poles / and larger shock-cord diameter. Wouldn't recommend scaling down any unless you have a much more precision printer than me.
Printing in ABS at without scaling up any might be a little sloppy, but it doesn't seem to matter much since the only thing it has to do is grab the knot and slip into the end of the aluminum pole.