For just throwing it together with "spare parts" you did really well. There are few guide lines for doing iwagumi. Each rock has a role to play. Although your arrangement has some concerns, it still boils down to whether or not YOU like it.
I quite like it and depending on what species you plant and where you plant them you may be able to pull it back towards center. The slate "cliff" on the left seems to be pulling my eyes to it and it alone. But that is just me. Possibly a much smaller rock placed in front of the pile on the right hand side towards the front might add a little balance. just my thoughts. thanks for posting it. look forward to seeing it planted.