This isn't exactly an adaptation, but wanted to mention that I bought one of these bags a couple of months ago, and don't know how I managed without it! https://www.ebay.co.uk/itm/225895336226?var=524984844194 I bought it for when I have to go to the shop or supermarket, because it's easier for a wheelchair user than any other kind of bag. It's big enough to accommodate my phone, purse, shopping list and keys, and holds them in the right position for quick, one-handed access. But it's also useful in the house if I'm trying to do things. I only really need to carry my phone then, plus the TV remote if I'm listening to the radio or a podcast. It saves me walking back to the sofa to find them when I get a messages or need to turn the volume up/down. It looks slightly bulky in the photo, but it packs completely flat and the fabric is very thin. If you had to lie down as urgently as I often need to sit down, it probably wouldn't get in the way. This design's probably best for a right-handed user. But there are types where you can attach the strap to either side of the pouch. ETA: for anyone who doesn't want to follow the link, it's a bit like a bumbag/fanny pack, but is wider and longer with a very much slimmer profile. You wear it over one shoulder and across your chest.
I tried something similar a few years back, probably a little bigger in that mine holds both a 700ml thermos and a small sandwich box, as well as a phone etc. I struggled with it TBH, it wouldn't stay put, stay where I wanted it to while walking. This may be because I am not a particularly small person and this type of bag tends to assume that everyone is So it got relegated and I went back to using backpacks of various capacities.
Yes, I think mine would slip down too if it had that much weight in it. But with just a phone, a small purse and a couple of keys, it stays put really well. In the house, it's only got the phone and sometimes a remote in it, and I forget it's there. It's unobtrusive because it lies completely flat against your chest and doesn't really weigh anything. I think if I wanted to carry a flask and sarnies I'd use a two-strap rucksack, which would stay put much better. You can even use this bag and a rucksack at the same time. I carry my music stuff in a small backpack, which I hitch onto my back even for the 20 metre walk between the car and the venue—I'm unsteady on my feet and need to have both hands free.
I used to use one of these at work (not waitressing) and they are really useful. Not good for going out or a wheelchair but may be useful to some around the house. Waitress aprons https://amzn.eu/d/g2WIa6s
