So I went back to the drawing board and came up with better approaches. Instead of the cigar shaped power bank by Anker, you can try using Jackery rectangular one for more flat contact attachment. Same mAh rating and also trusted battery brand as well.
https://www.amazon.com/Jackery-Smallest-3350mAh-Portable-Charger/dp/B00L9F95RO/
Another approach is to use a flat, credit card shaped power bank attached to the back of the top screen plate. This one is 1500 mAh, which will supplement 1x battery life to the N3DS. You might need to get a 2nd top cover plate and glue the battery flush for low profile.
https://www.amazon.com/dp/B07BWGT3PW/
https://www.amazon.com/ALLONE-micro-B-3DS-Adapter-Charger-Converter/dp/B0101UA2SS/
Lanyos battery + ALLONE adapter drawn to scale.
The MHXX has enough interior volume for internal battery mod, so 3DS users with this hand grip like us can keep the function of the kickstand. Sorry I didn't photoshop, got lazy. It's possible to add USB input and output ports at the top if you want to stick to cable charging. Qi wireless charging requires some new relocated pegs to keep the grip structurally stiff. I don't think many people would try this semi-hardmod due to risk of destroying $50 accessory that'll get more rare to buy as supplies dwindle.
This totally looks like the ultimate grip lol. But you're right, I most likely won't dare to try this as this could potentially ruin the mhxx grip. About it being rarer, I do think it is becoming rarer as nintendo have moved on to new2ds xl and mhxx grip for 3ds xl most likely wont be produced anymore(they are quite expensive on ebay and amazon too lol)