Haven't got a spare 26" inner tube but I could fit my 700x28c sideways into the middle compartment. The other two compartments hold two multitools, and it's still possible to fold both sides into the middle. I reckon you'd be fine, but maybe someone else can check.
@spelunky @AlexD @jonuxuk @sentiero can anyone verify if the tool roll will take a large inner tube, like an MTB 26" or 29"?
Thankyou