Apologies. 23 x 17 x 8cm are the dimensions of the one with the biggest surface area. I have a narrower, taller one too.Tedious is having to ask what size the lunchbox is, repeatedly.
I suspect a huge bar bag without any ugly permanent bracket will droop and spill the lunch too.
The bag from Amazon arrived today and it was crap. It had Velcro straps which wasn't the end of the world. However, I couldn't even get them unfastened because the Velcro went through the loop. That is on its way back this evening.
I think I might have to concede and get another pannier rack on my road bike. Under duress.