For custom moves, I tend to use them for elements of the environment or setting with which the PCs are likely to interact in a way that is dangerous, interesting, or advances the story. They should be cleanly applied, easy to interpret, have well-defined triggers, and have concrete mechanical or fictional effects.
There's no point making a custom move for something that's never going to happen, and custom moves shouldn't be created as an excuse to punish the PCs.
I find that in a lot of cases, the easiest and most useful baseline is to simply make use of act under fire. So for instance, your custom move could be, "any time you travel in the Ash Wastes after dark, you are doing so under fire." So if my character is caught out in the Ash Wastes and trying to get back to the hardhold after nightfall, I'll have to roll+Cool. On a 10+, I make it back, no worries. On a 7-9, something has gone wrong, and the MC is going to offer me a worse outcome, a hard bargain, or an ugly choice. On a miss, something bad is going to happen and the MC is going to make the appropriate hard move.
This act under fire mechanic is beautifully open-ended, covers a staggering array of cases, and is probably one of the easiest ways to implement a custom move, especially one where the characters are interacting with a dangerous environment. It also saves you the trouble of having to come up with lists if there are umpty-bajillion things that could go wrong.
When it comes to custom moves in which lists are involved, a good rule of thumb is that the options are all positive, and from them you pick 3 for a hit or 1 (maybe 2) for a partial. Try not to make people pick more than three things, because it gets onerous and time consuming - remember, you want to keep the story moving. Depending on the flexibility of the move, figure those picks come from 3 to 5 total options.
This is the general structure of many of the basic and playbook moves in AW, and there's a reason for that. Things ranging from read a sitch to seize by force to pack alpha work this way.
On thing to consider (and it's something that tripped me up a little bit early on when doing custom moves) is the idea of complication versus opportunity. By that I mean you should be asking yourself, "is this move intended to complicate the PCs' lives, or offer them an opportunity?" Essentially, it's a question of whether 10+ is simply a success without complications (as it is when acting under fire) or whether a 10+ has some additional bonus. Much of this is going to have to do with the overall level of difficulty or danger posed by the trigger for the custom move. If something is really difficult or dangerous, then maybe limiting uncomplicated success to a 10+ is OK. But if something is less difficult or dangerous, maybe you use the 10+ option to throw in an opportunity for the player, something extra they get for rolling well.
So for instance, if you want to keep the structure of act under fire but still offer an opportunity maybe your custom move becomes, "any time you travel in the Ash Wastes after dark, you are doing so under fire. Additionally, on a 10+, you find some mature Night Blossom during your travels (+1 barter)." Now there's a reason to travel in the Ash Wastes after dark, which will encourage players to do it, which will in turn increase your chances of being able to complicate their lives.
Just remember that the best custom moves are simple and straightforward.