That is definitely my preferred method "Box In Bag". Works especially well if you use square pots. With a bit of hunting you can find a cardboard box the right size for your required plant numbers and pot size. If you find good ones, the plots will fit so snug that they don't even rattle.
I use Waitrose green hessian bag for life bags. I have two sizes, one narrower but taller and one wider but a bit less tall. I have gone over the white embroidered Waitrose logo with permanent marker to make it a bit more stealth. Even so, it is still a largish shopping bag, which is a bit sus to be carrying on a footpath miles from any shops. (Taking some "insert innocent items here"
to a mates house or on my way home from a mates house officer.) I am planning on getting a 30 or 40 litre rucksack for this year as I want to look even more subtle. Tricky finding one wide enough without it being giant. Choice of bag is important. Fishing bags/boxes are ok, unless there is no public fishing near where you grow, in which case they are well sus. Still better than brightly coloured laundry bags, that's really dodge and would make me quite para, especially in the daytime. Big bags in the countryside generally look sus. Better to do less plants per visit but more visits than take a bag that makes anyone who sees you wonder wtf you're up to. Probably not such an issue in urban/semi-urban, but be careful not to look like a burglar.
I have 4 different boxes, each with it's own purpose. One for 2x 4" pots. One for 3x 4" pots. One taller 7" square box for a larger single plant. One 4" square for a regular size single, this I put in my small rucksack with a folded up cage, a hand trowel, a carrier bag of well nuted compost mix, a 1 or 2 litre bottle of water with PM Root Stim. I call this my single hit planting pack. I can go to a site on foot, prep it and plant it in one hit. I always reccy the sight first on a separate trip.
Boxes in bags look fairly subtle and hide the shape and smell of the plants. There are almost limitless reasons to be carrying a box in a bag. Make sure you know what you will say if anyone does ask where you're going. Make sure your excuse/reason
fits with what you are carrying. Boxes in bags feel safe, both from prying eyes and from crushing/squashing. You may think you can just stand plants in a bag and they will be ok. You are probably right. But, how many times have you slipped going down/up a steep bit or climbing a fence/stile or crawling through a hedge. Accidents happen, and if they happen when you're carrying precious carefully nurtured plants, you want to be sure they will survive unharmed. At least 3 times now, this method has saved my plants from damage during fence climbing / bramble tunnelling etc.
Keep those plants safe and those bags subtle.