Back in the PSP days i wasn't much of a fan of the Monster Hunter series. A friend showed it to me, projecting that almost overpoweringly emphatic gusto any seasoned player does to potential new teammates, but i couldn't get my head around the contorted control scheme required (affectionately referred to as "the claw" by people who actually LIKED it!).
Now that i finally "get it", having spent well over a thousand hours on the series since the release of Tri, it's almost serendipitous that the solution to the New 3DS's slippery control scheme comes straight off of the front of that stalwart PSP.
The problem with the New 3DS is that the C-Stick becomes slippery with use as the silicone attracts oil, making it nearly impossible to adjust the camera vertically. In Monster Hunter Generations, my current obsession, i'll get half way into a big fight, lose any control to look down, and have to run away to wipe the stick. Awful!
Being old enough to remember using old IBM Thinkpads with their very similar controller, i set out on the internet to look for replacement "nub covers". Those old IBM's got around this very problem by impregnating the rubber with a rough, glass-paper like material. I can't believe Nintendo didn't leverage that twenty years of experience!
Wondering if the square connector was the same size, i stumbled across a Redditor who found an even more interestingly compatible part, the PSP-1000 Analogue Cap! It was too perfect, i instantly bought a pack of four on eBay, for a dollar, WITH SHIPPING!
The process is simple enough; Remove the old rubber cap, and click on the PSP cap. Since i have a bunch of spare OG C-stick caps, i just pulled it out of the console, tearing the silicone anchors. The alternative requires opening the console, and i'm saving that for a more major operation later.
Once out you'll see a square peg which looks very similar to the one in the PSP or a Thinkpad (i'm pretty sure it's a perfect fit, too). If you really wanted to reverse this operation, trimming the ears off will let you push the original cap back on.
You can get these PSP-1000 caps in a few colours, but i thought white would be the best fit with my coloured buttons. The shipping took an age, but you get what you pay for. Just line it up and press it on.
And done! It almost looks "too flush" to work, but it's actually FAR more sensitive than the original. You only need to use slight force, it works properly in all directions, and it will never slip mid-battle. If you're playing Monster Hunter on a New 3DS, you absolutely NEED to do this mod!
Full credit goes to Reddit user "Spentthedayonreddit" for working out that compatibility!