You're right that a 12 inch square basket won't hold it for too long. My plant was growing in a 14 inch round pot..14 inches tall by 12 inches deep and was moved for permanent growth into a 5 gallon pot that had a 4 inch layer of drainage. I'm a HUGE advocate of putting styrofoam peanuts in the bottom of all my nep pots so the water just rushes out the bottom really fast. It was a solid mass of roots in pure LFS. PLEASE!!!! DO NOT USE PURE LFS ON THIS SPECIES. Yes, many people do and many are successful. But you CAN NOT keep this moss in good condition for the long term and it must be taken off the roots when it decomposes or you risk the loss of the root system anyways.
So many growers out there swear by this stuff. But how many of them are growing their plants in 2, 3 or 5 gallon pots of this rubbish anyways? They grow their plants, for the most part, in smaller pots. Usually under 8 inches. And those who do grow in larger pots of pure LFS have not been growing their plants long enough to experience the utter destruction to ANY Nepenthes root system when trying to remove it or just leave the plant in its old medium for who knows how long. And it MUST be removed when it goes bad. I've seen many plants from people who make claims that their plants are doing great in this stuff and it hasn't been replaced in 5 years. You know what? I've been around the block a few times. Their plants ALL looked like terds! They really don't know what a truly healthy specimen should look like.
Nepenthes rajah does NOT grow in pure LFS naturally and it experiences very good drainage where it does grow. It may be fine for smaller plants and even was good for my plant in its larger pot. But the moss was bad..smelled like it needed to be replaced, and in doing so I did some damage to the roots. I even did this under water to try to ease the moss away with a pair of forceps. Took me 2 hours to get as much of the moss away as I could and I still left quite a bit but the roots were completely tangled up in the stuff and I couldn't prevent hurting them. I had no choice.
So I replanted it into a 5 gallon pot of my standard mix; the stuff I use that all my other N. rajah are thriving in. It immediately went downhill from there. I knew it was destined for death at that point but I refused to put it back into LFS.
Please don't go that route. Trying to grow large neps in pure LFS in large pots and cleaning it off the roots when it is NECESSARY to do so can mean the ultimate destruction of your specimen.
It makes me cringe when I see sooooooooooo many testimonials from folks who grow their neps in pure LFS. It's even more funny how really newbie they are...growing the plant for 8 months to a year is not experience. Nor is growing it in a 4-6 inch pot. Oh Please! Let's see what happens when your plant is grown from a 1 inch seedling and is eventually, in time, moved up and taken from its 12-14 inch pot or larger (of pure LFS) and is cleaned and repotted. I do not use LFS for growing any neps, ever. Nor do I top dress pots with it. I only use it..sparingly and mixed with other components, for rooting cuttings. I'm sure there are growers out there who ARE successful in managing plants under the same circumstances...but they are probably way more experienced than I.
I appologize for my ranting. I was and still am completely traumatized that I killed my prized specimen. I could have prevented it.