I'm a big fan of the Printrbot Metal Simple; we've been testing them a lot at the office in order to release their driver. Now if you have Windows 8.1, you have plug & play support for this printer (along with Makerbot and UP!). However, one thing it's missing is a place to put the spool. This simple holder presses onto the tops of the Z-rods and holds all the kinds of spools we've bought from Amazon and Makerbot.
My shameless plug for any of you using Windows 8.1: Download 3D Builder and give it a whirl (it's free). If you have a supported printer (like the printrbot above) you can file -> print right from the app. Not to mention cool tools like Booleans, Embossing and Repair. It's been really fun working on this and the slicer that's now part of Printrbot's driver; I'm proud of them, but I can assure you we're still hard at work and you can expect major updates as we finish new features.
You can print this right on the Printrbot Metal with no support (you may have to rotate it to fit crosswise). Just press it onto the Z-rods as in the pictures. If pushing isn't enough, tap it down lightly with a hammer.
If you need to tweak the size for some reason, you can use Customizer for the primary dimensions.
Set the spool on the cylinder and feed the filament through the arm. If you want any advice on the Printrbot, or 3D printing in general, you can find a bunch of videos I did with my boss here: http://channel9.msdn.com/Series/3D-Printing-Essentials.