I think one advantage of string over glue is the amount of moss surface that’s held or bound to the wood. With glues, as the moss grows and moves with the water, or fish/shrimp/whatever, the amount of unsecured moss relative to secured moss increases. My experience is that it always breaks free, sooner than I’d like. It might also be that the part of the moss within the glue clump dies back, weakening the connection
But if it’s tied, you get a better connection between moss and wood.
In this situation, with your large piece, you could lay out the moss in a mat, lay a plastic flexI grid stretchy thing over it (sorry don’t have/know a name), and tie the whole thing down with loops of thread.