The Substrates tool suggests an MgO (100) substrate for a Li2Se (100) film, but the lattice parameters are very different:
MgO: a = 4.256 A
Li2Se: a = 6.020 A
Edit: Nevermind, of course the film just rotates 45 degrees on the substrate (4.256 A * sqrt(2) = 5.95 A). Score one point for the tool!