Hey, GG, you're not late at all. Thanks for the thoughtful, helpful ideas and input! I can't get moving on this until the house plants all go outside, hopefully just a couple more weeks. Can hardly walk around in here at all.
Corner braces are a great idea! Hurricane clips came to mind but there may be other similar doodads. The sides have 3 screws at each corner and are over an inch thick, but sure couldn't hurt. This is going to be full of heavy stuff, pushing outward for sure, and could get sideways (trapezoidal?) without them.
I'm nervous about ruining this planter way too soon too. I've used wooden boxes that were intended to be window boxes on the ground and after 2 seasons, they are mulch. Same with old drawers used for planters. The bottoms of those usually only last 1 year. But this won't be on the ground, and it will be dry all the time, except for some light spray during really windy storms. It will be under the roof of the porch, and have waterproof paint.
The bottom has about 100 drill bit holes in it. I think if it was lined with vinyl, stapled up the sides, I could poke through it at some of those holes, just in case, but don't plan to be putting enough water for it to run out. I'll be using rain water so a buildup of tap water chemicals won't be an issue.
Keeping it dry enough for succulents should make/keep it an unsuitable environment for mold. I only have trouble with that on the back porch where the sun never hits.
For potted plants, I usually use some kind of mix that drains so well that copious amounts of water must be applied all over the surface to ensure that the contents are moist throughout. But for this type of thing, I think a much more water-retentive mix would work a lot better because the way those soils wick and distribute water upward and sideways. Of course going all the way to a bag of peaty potting soil would be way too far in that direction, but I might buy some for the first time in a LONG time to add some to get the texture I think this needs. Or I may find enough suitable material already in other pots while repotting, stuff that's too broken down to suit "the big boys" but could be just the ticket for this.
The table doesn't look any different yet, but here's some of the stuff I've made. Chairs, a wind chime, don't like to wear that pearl pendant so put it on a wire to look like it's hanging from a shepherd hook, DH hollowed those corks out to be decorative planters, and the rocks I bought on ebay years ago, planning to string them for jewelry but never happened. So I strung them on florist wire and they keep their shape when bent, so they can be borders for little "flowers beds" in the mini garden, or along a path or something. Won't have to place each rock, or worry about them getting out of place, easy to rearrange.
