nostr:nprofile1qqsvrlrhw86l5sv06wkyjgs6rrcekskvk7nx8k50qn9m7mqgeqxjpvg8u2e5q built one https://shop.lnbits.com/product/nostr-signing-device
Also because the keys need to be hot all the time so extension/bunker style signer is most viable i guess, doing all of the signing required on hw signer and manually going through every request with the same level of scrutiny would be a nightmare