On Monday, June 25, 2018 at 9:28:17 AM UTC-4, Dr. Deb wrote:
On Sunday, June 24, 2018 at 10:57:34 AM UTC-5, Electric Comet wrote:
looking for the favorite planter box liner


or maybe i will just make drain holes


pond liner seems too stiff but seems to be the right stuff

You really have two choices, you can either let the wood of the planter box get wet, or you keep it dry.

Assuming the planter is sitting outside, its going to get wet when it rains anyhow. So you need breathing room around the liner to allow the planter to dry after a rain and you need a drain (or drains) in the liner that pass through the planter and allow the excess water to drain out of the soil and yet not keep the bottom of the planter soaked.

The construction is simple, battens on the bottom of the planter for the liner to rest on and on the sides to keep the give the wood room to breathe.. Then in the bottom of the liner, seal in one, or more, drain tubes and have them extend down through the bottom of the planter.

Your concept is spot on...Maybe the liner solution lies in a piece of drainage board, used for basement/foundation water control...
such as this:
https://www.lowes.com/pd/Hanes-Geo-C...A so6EALw_wcB

Probably find some cut-offs at a residential or commercial construction site