Splicing: I've made my own fids. Knitting needles work, as does aluminum tubing from the hardware store.
For the tubing, I think I cut a slit at one end and then folded/hammered the end into a tip, then filed/sanded that smooth. Did several halyards with that.
For the knitting needles, I cut them in half, then filed a flat spot. It helps with taping line to the fid.
Much of the double braid is pretty stretchy, easily gives an inch or two over the length of a mast. That means your sail shape changes with every gust -- in the direction you don't want it to: fuller.
I've been using a technique I found described on
L-36.com to splice double braid tails onto Amsteel (dyneema) halyards. Amsteel is really easy to splice - the technique is a bit different (Brummel splice) but easy to master. The transition to double braid isn't more complex than splicing double braid.
The exact numbers should be in that other thread, but I ended up using 3/16 for the halyard tail and 1/8 for the Amsteel. I can use 3/16 for the tail, because it's still grippy enough for the purpose of raising the sail, and it's not under load while sailing. The 1/8" Amsteel is plenty strong enough and simply doesn't stretch. When I have full hoist on the sail, it's like coming to a hard stop. I measured it so the tail starts right before the cleat for better holding (Amsteel is a bit slippery).
If you have 5/16" and want to use that as your tails, then you either need to go oversize on the Amsteel, or, and that's what I did on one halyard, step it down, by splicing a bit of 3/16" Amsteel as an "adapter" between the 5/16" and the 1/8".
As Carl discovered, splicing is fun - you may have to buy a second boat to have enough lines to practice your craft on