At the end of second year have Dobby claim the life debt that Female Harry owes him from when he saves her from Mr Malfoy.
Use the trope of a house elf always needing to be bonded to some one to survive but when Dobby attempts to bind himself to Harry instead of Dobby being the slave Harry is enslaved by Dobby because of the life debt.
Have Dobby embrace his position as Harrys master and do his best to become the best master he can be. And have the bond make Harry accept her position as Dobby's slave.
You can keep it as only Harry being Dobby's slave or you could add Other witches like Luna and Hermione.
Mabye have Dobby while trying to work out how to be a good master the the Great Harry Potter find a Magical BDSM shop and give him by a book on how to be a good master. And with Dobby 'learning' how to use Harry as a master should.
