This is a replacement filter / grate for a InvisaFlow 4400 Flex Grate Downspout Filter I purchased from Amazon.com:
http://smile.amazon.com/gp/product/B0061Q5DW2/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1
Our dog ate the original filters, and I needed to replace them. The 3D printed parts worked great. I also created a deflector to attach to the inside of the upper portion of the downspout to deflect water towards the back of the filter instead of out through the opening.

1) print parts. I printed the chevron side down on the build plate and had the vertical stripe reinforcements bridge over the top of the chevrons. That worked well for me, but your mileage may vary.
2) use zip ties to attach to the original downspout filter