This matter has been puzzling me for quite some time.
I don’t know if this has been discussed previously nor do I know how many font makers care about the ligature
łł for Polish. In my opinion it is an important one.
I wonder what might be the appropriate solution for the stroke. As
Adam Twardoch has demonstrated, in script style typefaces a single stroke above the l’s, possibly with a wavy touch, would be the preferable choice. But I’m not sure if this solution would also be the best one for e.g. mainstream sans or serif text fonts.
By now I can imagine these five potential models:

Which one would be the best?
Next question: is there also a special requirement for the capital version?
Comments
A Pole will have the ultimate answer, but in my opinion the whole thing is done to avoid confusion with the t, so only 3 or 4 is right ( is there a difference anyway? Ascender height should dictate how long you make the l's)
The sound itself is an L, but made with the cheeks, and is called ew. What I am more concerned about is the naming it in the most robust way, so it can show correctly in all or most ecosystems. Should it be lslash_lslash?
The second to fourth are not recognisable as anything.
The fifth is the best IMO, and preferable to łł
It is far better trust a font designer's instincts for what looks right and is easily legible. If in doubt, do nowt, i.e. don't add the ligature or make it a discretionary ligature
Thank you all for your opinions, dear colleagues. – And now a thread about the most useless ligatures in general?
I don't think confusion with middle welsh would be a major issue for most Polish texts.
do not forget about other "ł" problems, like a ł_k, d_ł :
But the copperplate capital find is a marvel. I will be dreaming of this.