1. To me using wood for sides of raised beds even with use pond liner is problematic. Wood rots quickly around moisture. There are fungi encorporated in a lot of potting and soil mixes that can invade an colonize wood. Consider using sheets of Galvanized roofing as a side material. Use pressure treated wood for the corner supports and the rail at the top and extend the Pond liner up to that wooden rail. Perhaps build on a flat area, use 12" x 12" cement patio pavers as a base to support your sip. ( I know, he said he has no problems with wood.)
2. Oxygen. Drainage tube.... it is important that the water level leaves an air pocket at the top of the drainage tubes. If you do build this, perhaps cycle it so that you allow some drying to occur every several weeks so that oxygen can returns to the potting medium. At the same time be careful your wicking medium does not completely dry out or you will have to surface water until you re-establish the wicking action.
3. Consider would put down a layer of heavy gardening fabric on top of the the tubes and push it down between the pipes before putting the medium on it. Consider using pure coconut coir at the bottom around the pipes and incorporate some into your medium. Lasts longer than Peat. Incorporate some of the coir in the medium mix you eventually use. Peat and pine bark mulch and miricle grow along with soil seems good.
Good luck with whatever you decide to do!
My sips. I use 5 gallon sips for some fig trees that I have so this is not quite analogous to what you are considering, but some of the problems are the same.
My 5 gallon buckets need to be portable because I need to be able to move them to overwinter the trees someplace dark and relatively cool but not more than 5-10 F below cool. I do NOT have a rain barrel or have an automated system.
I do use a manual timer with use drip irrigation on 1/2 pvc tubing to water them. I can also hand water through a covered pipe individually and fill an approximately 1 1/2 gallon reservoir created at the bottom of the 5 gallon bucket.
A picture. Holes are drilled at about 4-5" above the bottom of the buckets, lined the sides with the heaviest landscaping fabric I could find (Fuzzy side in), and put a Fabric covered inverted plastic collander inside at the bottom, with a pvc pipe cut at the bottom at an angle. The top of the colander is above the holes which allows drainage and an air pocket between the roots and the reservoir. The wicking material is tucked inside the bucket at the bottom in the circular gap around the outside and top of the fabric covered collander. I used Coconut Coir as the wicking material because it wicks well and lasts longer than peat.
