It's just coroplast. Actually a previously used piece which is why it looks lumpy in places.

I took a steel rod, I think 1/8" but maybe smaller, and laid it across the drop bars bending it around on the ramps. Twice, two horizontal struts. I can just slide them on and off the bars. The straps (just one strap over the stem, and one across each side to hold the bag in) are some elastic my wife had from a craft store, with velcro sewn on the ends of the stem strap. That's it for mounting.