I too am looking for an option/alternative to the aluminum accessory deck to my 2017 Radwagon. I have looked on Amazon, ebay, craigslist, and google to find options/alternatives/third parties.  The only place I can find it for sale is:

https://www.capuncless.ga/products.aspx?cname=radwagon+accessories&cid=5&xi=3&xc=21&pr=65.99

I have emailed them to confirm they have this product available but have not heard back. The website looks a little sketchy. Any ideas here are welcome.