i use a 58 litre topbox on my fazer and usualy fill it with 5kg-15kg of stuff most days...
Feel free to make a rack that doesnt flex and can handle it ill happily review it im happy to pay postage.
The 5kg weight limit would include the weight of the box as well, not just what's inside the box.
I realise 5kg is probably a little bit over precautious but if you Google "renntec rack snapped" you'll see it's a common occurrence.
I use the Givi monorack bars now which have two mounting points rather than just the one, and are certainly designed more to take the heavy weight of a top box.
You have to take into consideration braking, accelerating, braking, accelerating, speed bumps, pot holes, cobbled roads, going off a kerb, etc, the constant momentum puts a lot of strain on these lightweight racks, even worse still if you have someone resting their back on the top box!