CHESS BUT ALL THE PIECES (except pawns and kings) ARE THE SAME AND RANDOMLY GENERATED

so, this is a custom variant i am trying to make but i don't know how. so basically, for each side, there are 8 pawns, 1 king and 7 pieces that are randomly generated (they all have to be the same).

the seven pieces can be queens, rooks, knights or bishops. basically chess 960, but all pieces (except pawns and kings) are the same. the only way to get another kind of piece is to promote your pawns.

the thing is, i don't know how to implement this while making my custom variant. as shown in the example above, white gets all rooks, while black gets all queens. i just made this up, and i don't have a random generator for it. i need it so that each player gets the random generation, and they don't have the same pieces. 

if anybody knows, please help me.