Gone 1×10 and now don’t need the shifter bit on the left. Is there a Shimano brake lever only that has the same shape as its right hand companion?

In short, nope, they’re all significantly different in shape

Not that I have found, I ended up with a lovely mismatched tiagra RH Sti, and tiagra R400 LH brake lever…

I think cane Creek do a nicer shaped hood on their brake lever, just costs a shade more…

What about stripping the guts out of it? Doable or no?

Well that’s an option if you absolutely must recover a few grammes. If you just want to stop it flapping then you can either mechanically lock it with the drill-and-screw approach, or lock out with a cable and clamp and then use barrel adjuster to tension out any movement