Looking for inexpensive shortwave antenna for use with Grundig S350 AM/FM Shortwave Radio

Simple wire can make a great antenna, and it's relatively inexpensive. I've had the best shortwave and AM reception thus far using a long loop (actually a rectangle) of copper wire as an antenna (but folks shouldn't try a loop with some radios designed for use with powered antennas without inserting an appropriate capacitor in series). If you do try wire, you would want to try to make the loop shape as large as possible, and connect the ends of the wire to your radio's antenna connectors.

You can also take any metal slinky and solder a single alligator clip to each end of the spring coil after removing the small metal clasp at the end of the coil of wire. This gives you a collapsible wire antenna that you can clip to a drape, or any hanging item using the clamp. And the other end can easily clamp to your telescopic antenna with out scaring or damage.
It works horizontally or vertically as an antenna. And you can add an additional slinky rig to make it even longer.

You can suspend it by hanging it off a balcony, clipped to a tree limb, a table umbrella, or with a fishing pole inside as a vertical antenna support.
Or along a simple string or cord as a horizontal antenna support.The original slinky box still makes a handy carrying case for your travel antenna to fit into as you travel.