If you are thinking of recording video as well, here's an idea -
https://www.bhphotovideo.com/c/product/1441156-REG/zoom_zq2n4k_q2n_4k_video_recorder.htmlThis is supposed to be released this month (note this is the 4K version of the previous FHD version of the Zoom Q2N). It is unique in that it's intended primarily for recording gigs in stage lighting, and unlike almost all video cameras it has a proper audio recording system with mics that are not an afterthought buried in the body. I would expect it to have the audio quality of a Zoom H1N. But it also has a mic input socket. A few days ago I recorded a jazz club gig using the H1N with Roland in-ear mics, and the result was, in my view, very good. I would expect similar results from the Q2N-4K (but can't guarantee it as it's not actually out yet). I ran a separate video of that event using an imitation gopro, but that came out pretty badly as it didn't handle the high contrast of the stage lighting. One musician wearing a white shirt came out looking like a flat white blob. The Zoom camera should - repeat should - substantially avoid that sort of problem as it's designed for that kind of scenario.
In addition to the recorder and an external mic, you'd need a rechargeable battery pack with enough capacity to cover the concert. Using internal AA batteries you only got 30 mins running time! You would also need a very fast micro SD card to handle the 4K video. Also, the camera isn't stabilised so to get wobble free video, you would need some kind of stand - maybe a monopod with a tripod foot which wouldn't require any space, or even one of those oversize selfie sticks.
The advantage of this rig would be that you would have,
at the price point, a good video outcome, a good audio outcome, and with the 4K video you could record a static picture at the time, but then in video editing software you should be able to pan and zoom with the benefit of hindsight to produce a reasonable 1080p edited outcome. The whole outfit would almost be pocketable. Not a stealth rig as the use of the camera would be pretty obvious, but no more so than a phone, but you've indicated that it should be permissable at this gig.