I have a Hardin but no QR sacred seal...

I'm gonna assume you don't have vengeful fighter which is basically better since it'll free up your seal slot to let you run like close def or distant def.

Since you're using Bold fighter (I assume) you can still give him either of the two since Bold fighter only works during the player phase so giving him a defensive boost in the enemy phase can keep him tanky. Armored Boots is another one though if you want extra movement since it works well with Bold Fighter considering it's nature.

There's really no substitute, until they come up with a Bold Fighter seal...
The point of that build is that it's the combination of both BF and QR he can double on both phases.

But fear not. It'll probably be forgeable soon enough. I believe Heavy Blade is forgeable now and that got released like a month ago? Two months ago?

You can use something else no problem, it will just be a different kind of build.

You could even give him QR2 for now, and start building him for mixed phase for the inevitable forging of the QR seal in a month or two.