Attachment was nothing fancy . . . . I just used the two flat steel pieces that I had originally mounted to the spoiler when I had used it as a flat detached wing on Black Widow, and just bent the straps 90 degrees to use as an attachment for the grill block. They ended up looking like this:
3 in. Zinc Plated Corner Braces (4-Pack)-15307 at The Home Depot
I pre-drilled a hole in each of the steel straps to mount to the inside top of the bumper opening, then pre-drilled a matching hole in the bumper. I removed the straps from the spoiler, bolted them onto the bumper, and then re-attached the spoiler to them.
I'm sure you could get a spoiler like this for cheap at any U-Pull junkyard with an older Legacy wagon in it