Maybe something like this.
https://www.sportsbikeshop.co.uk/motorcycle_parts/content_prod/279236
I have one, stored in my topbox, but i attach it to the rear seat if I need extra capacity. Have a look around can probably get it cheaper.
The only downside to this one is that it is zip on so some aspiring footballer might have it. but rather a £20 bag than the MT.