There are lots of m40x series (m401, m402, m403) mods out there for both the standard and XL cartridges. Do a few searches for m401 cartridge mod(s) and you'll find them. The m40x XL cartridges are not much bigger than the standard length cartridges, but I understand their e-liquid capacity can still be as much as 33% more, which is not bad for the extra 3mm of length (though still on the small size compared to other model e-cigarettes). They don't fit in m40x PCC boxes when attched to a battery and atomizer (regardless of battery length/box size apparently), but if you trim off the top of the silicone caps they come with they will fit as spare cartridges. Even then, I still carry a little 10-16ml sampler size bottle of e-liquid in my pocket to regularly top off my cartridges with while I'm on the go. Works out pretty well. The only "modding" I did to my XL cartridges was buying them already factory pre-made with the PTB (Pyramid Tea Bag) mod. Sadly, I haven't seen that optional available anymore anywhere for awhile now, but the benefits of that particular mod were fairly minor anyway.