Aerospace tig welding tests almost always involve a fixture like the one shown in this video. Little details like how you place the chill bars can make a big difference.
Even though it would be rare to weld plain 304 stainless for aerospace, the tips still apply.
Typical test material for the stainless group would be 15-7 or 17-7 stainless steel which is more difficult.
This Video gives tips for TIG welding a 1G flat butt joint and how to watch the puddle sink to make sure you are getting penetration,
A tee joint fillet weld test in 11 ga thickness usually requires penetration into the root but not all the way thru the back side
