A new aluminum Markins L-plate LN-FZ for Nikon Z mirrorless cameras was announced in Japan (specifically designed for FTZ adapters). The suggested retail price is 19,980 yen including tax. The dimensions are 67 × 45 × 79 mm and the weight is 67g (via DC.Watch). Markins L-plate for Nikon Z are sold on eBay (they announced another version a few weeks ago)
Check out also the Nikon Z L-plates from Really Right Stuff and the Kirk BL-Z L-bracket.
More third-party L-plates options for Nikon Z cameras are available on eBay. Additional discussions on this topic can be found at the Nikon Z6/Z7 Facebook group.