Like most animals worth under $5 million, the shower lions can afford is made of plastic.  The problem is chinese put magnets in shower curtains assuming everyone in the world can afford metal bathtubs like they can.  Many tons of the Mokelumne river have flowed into the ocean needlessly while lions struggled to keep their shower curtains water tight.  

The starting point was attaching magnets on the top with strap things.  Attaching magnets or metal to the shower is going to be much harder.  There is a way to attach magnets to the bottom with weighted hook things, but new magnets would be needed on the curtain in those places.  Magnets on the sides may only have suction cups.  These ideas don't provide a water tight seal.

Another idea was a 2nd rod which presses against the sides of the curtain.  It would keep the sides pressed flat & would bend outward to keep the center pressed outward.  It could be made of cheap carbon fiber rods connected together like trecking poles.  Those have a long bungie cord on the inside.  Other ideas are a bow & arrow type rod or a tensegrity structure.  

There could be an ordinary PVC pipe with a CF rid fitted inside.  A string inside could extend the CF rod.  It could attach to the outside with a knot.

It has to be light enough to be supported by the curtain, so 3 CF rods with end fittings allowing 1 rod to slide.  A common fishing line could pull all 3 CF rods out.  The fishing line would hook onto something.  It has to be easy in order to escape from a spider.

The winner was a simple spring pressing 2 6mm rods into the sides of the curtain.  The mane problem is it doesn't collapse.  It's slightly more painful to store but very simple to set up.  The rubber feet are essential to make it stay in place.  The length of the shock absorber depends on the overlap of the 2 CF rods.  These were the only 2 rods around.  They came from a local hobby shop over 12 years ago & were intended to replace helicopter skids.  The spring came from a Tamiya lunchbox suspension & is waterproof.  The spring compresses from 70mm to 40mm & the rods overlap by 170mm when compressed.