Easy fit a 23.5cm pump in a roll. But fek me £40 notes is a bit steep for a pencil case, swear I only paid half that when I got it.
I use the bagman QR but with a bit of an alu strip bodge rather than big wire pictured above, but I already had one for a camper bag.
You can also get seat post adapters for klickfix barbags, they work pretty well at the back.
On the cheap you could knock something together using a gas mask bag from an army surplus?