Yield has free flash minting of fyDai, and very low fees for fyDai/Dai trades that are close to maturity. You can combine those for cheap Dai flash loans.
To use it you code a contract that inherits from it, deploy, and start flash borrowing. 💸
I deployed a dummy flash borrower in Kovan, that you can use yourself to practice. The fees are paid by the contract, so pump it with some Kovan Dai first!