Hi,
I want to make a planter from deckin boards which come in 4" or 6"
widths.

The whole thing has to sit on a small wall 9" wide which retains our
lawn (ie the wall is made from 9" bricks).It's to stop our toddler
running off the edge and falling onto the patio.

Would using three 6" boards be too small , ie one for the base and one
for each side?

Alternatively, I could use a 6" for the base and two 4" boards on each
side.

Not sure what we'll put in the planter, probably some type of small
shrub , any suggestions welcome?
Thanks,
John