Probably our best ever back up keeper was Steve Ogrizovic, who only played for us three or four times in five years but went on to be really impressive elsewhere.
But another part of the problem we and all other clubs have now is that the back up keeper doesn't play in the reserves. Karius should have been playing week in, week out for the reserves, with Achterberg holding long and thorough analyses after each performance, going through every single aspect of the game and his performance. That's how he'd improve, if he can improve - hard work in low profile games, not coming on to be booed and groaned at from the first minute in major first team games. No outfield player would get a chance in the first team simply because he hasn't played for a bit - he'd have to do what, say, Ings is doing, playing in the ressies, getting match fit and striving to impress. If you're a keeper and you currently have problems with your kicking and general distribution, or your punching, positioning, whatever the hell it is, then you need real and competitive match situations to really work on that and build up confidence. These days we just bung the player into the first team, confidence at basement level, and hope that he gets an easy couple of games to get settled. It'll be more through luck than judgement if that ever works. I'd suggest a solution but I don't see one.