DREDGE

You'll need to fully upgrade your ship to get every slot possible. There are 4 total slots for lights. 8 total slots for nets/poles, and 10 total slots for engines.
After that, you just need the money to purchase enough equipment to fill everything and the 20G is yours.
A great place to farm lumber, cloth, and scrap is just to the north of the Steel Point dock. There are 3 dredging locations there, one for each of the materials. After you've emptied those spots, simply dock, save and quit from the pause menu, and continue. Rinse and repeat until you have all you need.
The same process can be applied to the refined metal, which you'll need to upgrade your hull. Find one of the 4 Traveling Merchant locations (the Rickety Pontoon in Twisted Strand, the Charred Pontoon in Devil's Spine, the Starlight Pontoon in Stellar Basin, or the Dusty Pontoon in Gale Cliffs) and go to the Shipyard. Navigate to the last tab on the right (the one with the picture of a bottle and board). She only has one piece of Refined Metal each time, so again, buy the metal, save and quit, continue, buy the metal. There are dredging locations that have refined metal, but you'll also need explosives to access them. Money isn't too hard to come by, so whichever way you want to accumulate the materials.