You make your own arrowheads? That is SO COOL!
Regarding substitutes for jute...
If you're ONLY looking for tinder, try this:
https://www.ebay.com/itm/Flax-tow-fiber ... 2827668807
Various sellers on etsy and eBay carry the same stuff - I just happen to have purchased from this seller before with good result.
If you ALSO want it to be cordage, this might work:
https://www.etsy.com/listing/599080095/tow-linen-yarn
I've never tried it, so I couldn't say for sure. But it looks like it'll unravel into a decent bird's nest. I see that seller sells raw tow as well, so perhaps they could say?
Regarding fishlines: totally beyond my expertise, but I did just see a reference in either Tudor Monastery Farm or Tales from the Green Valley (both on youTube) to plaiting fishline from horsehair: apparently it was quite the thing at one point.
Finally, regarding "things of elvish make" --
Oh, I have been here.
First, please take this in the gentle spirit in which it's intended.
We all do this hobby because we want to have some goofy fun, and I know some among us are here explicitly
because they're tired of the stitch-counting stickupitdness that can exist in historical reenacting. There's no jury board on MERF, no standards - not even an agreed upon common mental image: we're all just here for fun.
Now that said..the way you're using the "elvish make" phrase indicates we're in a very familiar territory.
Everyone who's done reenacting has been here and has stories. I have SOOOoo been here.
There are two approaches to adding things to a presentation.
The first approach begins with an item one really thinks is cool or useful, and asks "how do I justify this?"
"I want to carry a burning lens - it came from Elrond!"
The second begins with a particular need the represented individual has, and asks "given what we can find in the sources, how would he solve this problem?"
"I want a second way of starting fire... what do the scenes I can find in the books tell me? What was done in analogous cultures?"
The second path means occasionally passing on things you've really really got the sparkle-eyes for... and that's hard.
But it does push you to learn more, to study more... and over time, one does end up with I think a coherent whole that you're
much happier with.
I really really don't say any of that to squash your fun. DO WAT U WANT.
But it's said from a place of having
tons of hours in projects I've either abandoned or need to remake because I have not done this well myself - and would like to save you some of the trouble.
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PS - none of that is passing judgement one way or the other on the particular question of the lens: I have no idea there.
I'm purely talking about mental processes.
PPS - none of the above applies to things one has to carry for modern reasons / reasonable safety precautions.