All there in the title. The most common, readily available is made by somebody called Carlson, like this one. https://www.basspro.com/Search...ine%20benelli%20nova I would hope that short cut-off spring is for illustrative purposes and is actually a single long spring. I also don't think it would take much knocking around, but I don't expect that kind of abuse.

Look over the Choate Machine and Tool website. They have extensions for the Nova.

^^^ Did that. They look substantially like the same product, but lower cost and greater selection of length/capacity. Under strong consideration.

The important thing with the Nova is that the follower is specific to the nova, a universal follower wont feed the last round reliably. Nordic makes some nice ext tubes if you have not already pulled the trigger.

I'll second Nordic products.