This is more a proof of concept than a recommendation.
I bought 2.0mm thick nylon fishing line to use as low-D, but had to make some major modifications to the instrument:
1. Enlarged the hole at the 'headstock' using a drill.
2. Enlarged the hole of the tuning peg because the 2.0mm string wasn't fitting.

I don't particularly recommend this as you need to make risky, irreversible mods on the instrument, but it worked out for me as a proof of concept. Fluorocarbon is probably a better idea for unwound low D string as they can achieve thinner diameters at higher tension.

For my personal use, I'm happy to leave my instrument tuned this way. The "thuddy" nature of the low D string almost makes it feel like it's coming from a separate instrument playing the bass/rhythm line for the ukulele.

D: 2.0mm fishing line (250LB DFS MONO CLEAR NYLON FISHING TRACE LEADER 2.00mm)
G: Fremont Fluorocarbon Low G
B: D'addario Pro Arte clear nylon classical guitar string (3rd, G)
E: D'addario Pro Arte clear nylon classical guitar string (2nd, B)

Amplifier: Roland Mobile Cube

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