Strongest Shield = Raiton no Yoroi + Third Raikage's durability. (Hell Stab is a different matter all together, but the parent technique happens to be RnY.)
According to rules at the narutoprofile wikipedia, you can't claim elemental techniques yet we already have few techniques listed in there that happen to be elemental related; this technique included.
Unless someone can show me a citation from the original naruto wikipedia, where it states that the Strongest Shield isn't exactly as I just described it. I'll gladly rebuke my comments, but until then there really isn't any prerequisites to claim it as Eric stated, other than having RnY; which means anyone can legitimately claim it.
"Like his son, A possessed high taijutsu prowess, being able to defeat several shinobi in one move.[16] He also had enormous speed and strength, being able to dodge Naruto's extremely fast Wind Release: Rasenshuriken and punch a rubber ball several metres away.[17][18] A's most impressive and renowned trait was his extremely durable body, earning the name, the "strongest shield" (最強の盾, saikyō no tate). With it, A could withstand nearly any type of technique,[11] and easily brush off attacks that actually damaged him, even those of Wind Release.[19][20] He could even survive being transported by the life-risking Heavenly Transfer Technique, a feat replicated by few individuals.[21] Ultimately, the only attack strong enough to seriously injure him was his own, resulting in the scar on his chest."This excerpt makes no mention of the RNY being responsible for his Strongest Shield. This isn't a technique, per se, it's simply the state of one's body. In this one:
"... Like his son, A can utilise the Lightning Release Chakra Mode to further augment his already renowned physical prowess and taijutsu." It's stated here, that the RNY simply further augments the
already renowned physical prowess. Meaning, it's more of constant, grueling, quite severe training regime... all physical.
I'm not sure who the first person, on SL, was the one to possess the Strongest Shield, but Athos is infamous in having it. When one thinks of the Strongest Shield, they think of Athos, considering the history.
I figured it was kind of like a yang release earth spear kind of deal. It's not regenerative and has no elemental weaknesses per say, but its strength is not infinite and it would be difficult to summon things (unless the player has a way to draw blood in another way). It's main strength is against hard fist and wide area blows, while it seems to be weaker to very strong, precise, pinpoint strikes (hell's stab one finger nukite). It is not a KG in the most general sense because the 4th Raikage did not inherent it, unless its recessive.
To my knowledge, there are no rites required to claim it.
Like Eric said, wild physical attacks are next to useless against it, if extreme strength isn't behind it. Now, if that strength is present, then it'll take time to wear down the defense of the Strongest Shield, comparing it to the Man of Steel's defense. (I could be wrong about this though.) Those of the Hyugan variety, internal attacks, are the best way to combat someone with the Strongest Shield. I'm not sure of the other fingers, but the One-Finger Hell Stab definitely works. Thinking of the Earth Spear, I'm not sure how comparable the two of them are, but I would say Strongest Shield > Kaguya Bone Armor > Earth Spear, because the former is passive while the latter two needs to be activated. But, saying that there are no rites to claim it could be false.
"A inherited his father's immense durability, having survived being transported by Mabui's Heavenly Transfer Technique unharmed.[32] His pain tolerance is high as well, as he neither flinched from having his left arm burned by Amaterasu, nor from cutting it off himself.[33]"From this excerpt, we learn that the Fourth Raikage inherited his father's immense durability. So it could have something to do with the DNA, perhaps the skin has a better chance of growing tougher through the training. I would say it's through the DNA since, Might Guy nor Rock Lee, have not been stated as having something called a Strongest Shield, despite their own physical training.
So, in my opinion, one would need to, at the very least, seek Athos for training to acquire the Strongest Shield. His sons, Ryoji and Tobias have the Strongest Shield, something that could easily be answered through the DNA route. Keito and Jay has the Strongest Shield, that could be explained through Athos training them personally to force their skin to achieve the level of durability needed to be referred to as a Strongest Shield. (Keito from being such a close homie and Jay from being a student.)
While Buffs like RNY, Sage Mode and the like can increase the effectiveness of one's Strongest Shield, they are not responsible for it's development or individual strength.
But, that's just a theory.. A Shinobi Theory, thanks for watching. Perhaps, Athos can add anything to this, since he's the one that had it the longest... cause he's old... like really old.