Conclusion
What can I say? I am in love with making paper by hand. This is the splashiest, messiest, most amazing process!
Although I'm disappointed that none of the paper I've made so far works well with fountain pens, I'm planning to keep experimenting with the fibers and different amounts of sizing. I will make paper that works with fountain pens!
My main takeaway is this: if you're at all interested in paper making, I highly recommend the Carriage House Papermaking Kit. You'll receive a well-made mould and deckle that will outlast any makeshift one you create on your own. You're provided felts, sizing, and fibers so you can get started quickly. This kit made it possible for me to make quality paper (and lots of varieties of paper) without any experience at all. I can't wait to see the results after a few months of practice!
(I purchased the Carriage House Papermaking Kit with my own funds.)