I've been buying (and selling my) camera-bags for years, (ThinkTank, Timbuk2, Lowepro, Dömke, Crumpler, brandless, you name it) only to discover that the best bag, the most practical one anyway, is the one I sort-of designed (for) myself in the end.
My main camera is also a m4-3 OMD by Olympus, which is really tiny compared to the big Canon full framers.
I took a really old lightweight smaller messenger-type shoulder-bag I don't even know the brand/model of, I think it's made by a German brand that also made tents of the same olive-army colored fabric, I got it in the early 1980s. Looks a lot like the one Jack Bauer wears in the first season of the 24-series. I had been using it to put my laptop (lenovo x230) inside, with a padded neoprene sleeve so it was safe no matter what bag I put it in, as long as it would fit against the back of the bag.
Then I put a foam bottom in the bag, which I got from some computer-part packaging I could easily cut to what I wanted for the rectangle-sized bottom. The rest, lenses, camera, I put inside several well-fitted neoprene pouches that I just put on top of the foam bottom, this was pretty good for my usage outdoors. A year later I extended the bottom-part upwards using a silicon-glue and a thinner sturdier foam (also sourced from some packaging material) so the inner sides of the bag would be a little more protected. Mainly because the lenses and camera would bounce around a little too much for my taste. Lastly I added 3 small dividers of a thin strong nylon material, also glued with silicon kit, and with that, I must add, it all has been working out just great for me. I don't really care about the looks of the inside of my bag, but I did use lighter colored foam and dividers, so it's all easy to find stuff inside.
The bag now weighs only 850 grams, all-in (but without content), which is still less than even the ones sold als 'lightweight'. Plus, it fits my laptop when I need it to.
While writing this I found someone who did a similar thing without extra padding: https://www.youtube.com/watch?v=4zNbJJTA-AY