The only way to show that a betting system is profitable is to have 1000+ bets with FLAT stakes and to be with a positive balance.
I don't think it has to be flat stakes. Every reasonable staking system is fine - don't forget that you lose some value by going with flat stakes all the time. If a system is going +/-0 with flat stakes, you can choose to fine-tune your staking and not go flat anymore and might be in profit with the same system (or sit at a loss, if your staking is bad).
Why do you think it HAS to be flat stakes ?