There are quite a few different makes and different sizes.
From small 'waist bags' (bum bags) to hold change and a few documents, to larger bags that will hold quite a bit more.
Try hiking/camping shops.
Think laterally and don't forget fishing shops, fly fishers stand in a river, can't put their bag down and so have it slung on their chest/belly where they can get into it easily.
Take a look on Amazon, you may find just what you are looking for:
https://www.amazon.co.uk/s/ref=lp_11...id=11967998031
Here's large one, meant for fishermen: (The fold down tray bit looks as if it may be handy)
https://www.amazon.co.uk/dp/B074R81Q...2-393a41a5be24
Another large cross-body one that can be worn at the front (diagonal strap as requested?).
https://www.amazon.co.uk/dp/B074FTH5...detail_4?psc=1
This one, although a backpack, swings round to the front to sit in your lap, and can be clipped/carried at your front. (It's a bit expensive but should last years).
https://www.wolffepack.com/pages/learn-more
This one's a bit of fun 'beer belly' bag:
https://www.amazon.co.uk/Dad-Bag-Adj.../dp/B075VQ8C7J