The old ARRL handbooks always recommended boiling wooden feedline
spacers in paraffin wax for weatherproofing. I suppose the idea is
that it's water-repellent. Don't know how long this can be expected to
survive weather extremes (ice, heat). In my experiments in my garage,
the wax coating seems to pretty much rub right off with my fingernail,
so I'm not sure how much good it does where the feed wires or tie
wires abrades against it.

I think if you're using an older transmitter - keep up the retro theme by
boiling the wood spacers in parafin like the old book says. (But I would
bake them at 190degrees for an hour to remove all moisture first.)

I would also be tempted (considering your home-making the feed line anyway)
to try what I recall being called "G-line", where on the transmitter end of
the line you keep increasing the spacing then just drop one line - and you
have only one conductor going up.
then of course, you do the same thing at the antenna end, going from
2-conductoe wide spacing to 'normal spacing'.
Experiment.
Report back on how it went.