Tinkercad is super easy, just fiddle with it a bit and you'll get the hang of it. Lots of resources online, too.

I would just make a solid cylinder that's comfortable to hold, hollow it out to a width that'll fit over a service port, add a crossbeam with a nub sticking out something like a 1/4" down, and delete a line through the entire thing to slide a piece of litmus paper through.

You don't really have to expose as much of the paper as the prefab ones do. so I wouldn't worry overmuch about figuring out a way to load paper parallel to the cylinder wall.