Bungee cord, the fabric covered cord, is terrible or launching gliders. I don't recommend it at all.

Also you say "small diamater" which means it probably is too weak.

You need Latex tubing, typically surgical tubing, which is not fabric covered. This allows the rubber to stretch up to 3X its rested length. Go back and read the first post and you will understand.

Silicone rubber can be used too as long as it is not fabric covered..

Note that in some parts of Europe, uncovered latex tubing is also called Bungee, but in the US, this refers to a fabric covered elastic which is not well suited for launching thermal duration gliders like the Aspire.