There's a few here - https://www.thonk.co.uk/product-category/hp-size/0hp/
And here - https://pushermanproductions.com/?s=workmate&post_type=product
But nothing Stereo.
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.