I use a small backpack all the time in boy mode.
For years I used to have a male wallet that I used with that backpack, and a much larger female wallet that I used with my girl bags (some of which are also backpacks or convertible). When the boy wallet was falling apart, I had a middle aged male customer show up carrying a similar style of girl wallet as I had, with no hint of being trans or anything being odd about it. Within a day or two (I forget), I emptied my boy wallet into my girl wallet, and it has been the wallet I've used virtually all the time ever since. The only time I used a different wallet was when I was going out en femme and needed to use a smaller one that fitted into the purse that I was using.