I have an MM and took the suggestion of a tpfer. Use 1 lobster-claw clasp on each side to fasten the metal hooks together:
Use a big lobster claw clasp or any other big clasp(from a keychain, or just buy one from craft place)
On either side of the Neverfull, there are the leather strings with little d-ring metal pieces at the top of the leather strings that attach the strings to the bag. You just take the charm or the lobster claw and clip it to BOTH of the metal d-rings on EACH side of the bag to hold them together which in turn makes the Neverfull more narrow since the sides of the purse can then be tucked in. The leather strings have a leather slider that was suppose to accomplish the same thing about narrowing the purse sides, but the leather slider does not stay up very well.
Here is a thread with pics showing a charm used to clip the sides together (you will need two charms or two lobster claws) but you will need to wait until later to view the pics since picture attachments have not yet been transferred to the new tPF servers:
I use a Purseket in my Neverfull MM when the purse is carried like a tote and it works great. But, I think some people like the Neverfull to look more narrow, so that's why they push in the sides by using the leather slider/ties to make the bag look more narrow. So, if you prefer this look, the Purseket is not going to really work inside the Neverfull since you would want to "stretch out" the Purseket to get full use out of it inside the bag.