Edit: I just realized there are a whole bunch of videos from people who have completed the challenge that I can go watch to find my answers instead of posting here.  LOL  So nevermind.
Well thanks to your tip, I was indeed able to make a Mesh, and thus, make the Mesh Sieve, but I don't know how to "turn it in" so I get credit for the first Achievement.  Doesn't this Skyfactory Challenge thing come with any instructions?   LOL I guess it's a map for "experts only".
And I hear ya about Skyblock modpacks, they're not really my cup of tea either.   I prefer to have a nice landscape to view while going about my business.  Just my own personal preference, most of my friends LOVE the skyblock stuff.  To each their own.  
But I like the concept of speedrun, because it's a way to practice getting out of my worst habit, which is to be a packrat.  LOL  Speedrun means you have to learn how to be optimal with your time, and learn how to not hoard items - only get exactly what you need, and no more than that.  That's good practice for any modpack, at least for me, because I tend to get bogged down when I play, my progress slows to a crawl, and I lose interest at that point (because I never seem to get anywhere LOL).