I haven't heard anything good about the state of New55 recently, and as noted above, that's 4x5 singles (fit a 500 or 550 back, won't work in pack film cameras) -- I honestly don't think we're going to see P/N or any other peel-apart film again beyond last-ditch, use-up-the-leftovers operations like One Instant (and at those prices, you have to be silly rich or really obsessed).
If you'll stretch your definition of "instant" a bit, and don't mind "portable" meaning "built into a cart", you could set up an Afghan Box Camera. Those are close to instant (ten minutes or so to a washed and drying positive, not counting setup time for leveling and filling trays when you get to where you're photographing, and a little takedown time to dump and rinse the trays and collect/protect any negatives or prints you want to keep), give a negative as an intermediate (positive is obtained by rephotographing the paper negative), are pretty cheap to operate and don't cost a bunch to build (need a large format lens in shutter, some lumber, some knowledge, developing trays, and a pair of old jeans or changing bag to supply a sleeve).