Cross Stitch

If you can't figure out a way to attach interfacing you could try putting the garment inside a zippered pillow case before washing it as is sometimes done with delicate pieces. That would prevent the agitator in the washing machine from touching the piece directly while still moving it around sufficiently to get it clean unless it is really soiled badly.

Or you could just handwash it, but who is going to want to do that more than once?