Yup, got your very own Mouse Door there......
Seriously, all can think of is maybe someone put a floor-button Hi/Lo dimmer switch there. I never had a floor mounted switch so I don't know, but you can certainly put a patch over it.
As for the Volvo (or CB) fluid reservoir, they fit the VW master cylinder like a charm, don't leak, stay water and dirt tight and seem to be a perfect fit for our application. I think Dr. Clock uses them, too, and yes, all you do is push it onto the rubber grommets and you're good to go.
Looking good! What's the road-ready ETA??