You can use Ronstan closed thimbles - they are compact and strong. I've seen them modified with a stainless bar or two welded across the narrow end of the thimble when there was a worry about deforming.

As for splicing to a shackle or a lark's head hitch - a small d- shackle can usually poke right through a small eye spliced in double braid line - Wichard makes some high-strength versions of small schakles that would do the trick. A large eye in double braid line can be awkward to use in a larks head - the line is just bulky -

Remember that spectra lines will creep under load (that's elongation, not stretch) at first, and you should allow for that - if you leave yourself too little room for expansion, you'll quickly find that you are too long.

Finally, an option for attaching these types of lines to a mast is to use a t-bar which has a ring or eye on the end instead of a swage socket or mechanical fitting. Norseman/Gibb makes them for attaching runners and checkstays - we have also custom welded them up in smaller sizes for dinghies. T-bars, obviously, need a hole in the mast and a backing plate, so it's a fairly permanent set-up (i.e. not so good for experimenting), but it's very clean.

best of luck,

jason