Forgive the terrible ascii art, but.....
Cut them to width for your cabinet.
The flat side attaches to the top of the cabinet and the underside of the speaker panel
The curved side goes against the marquee
When you look at it head on, you just see the two curves.
If you looked on end, it would look something like this:
---------------------
| | \
marquee -> | | \
| | \
| | /
| | /
| | /