Yes, tons of DYI options out there with wood, and the idea to raise them off the deck is easily done. I would suggest one of the following options:
Treated 8' long deck boards, (can make two four foot boxes with same,) or treated 1" x 6" fence boards, and a couple pieces of 2 x 2 or 2 x 3 for framing. On the bottom, put two risers to keep it off the deck on a four foot long Planter Box. Drill drainage holes wherever you want. CUZZX has a beautiful one. Cheers!