I mistakenly installed a post protector sheath on bottom of my 6x6 post without drilling the drainage holes. I’m not sure why manufacturer doesn’t include the drainage holes to begin with. I buried the post and backfilled with concrete. Top of post protector sticking up from ground about 12 inches. 100% unwilling to dig up the post so what is next best option? Seal the border/gap between the post protector and the wood with exterior grade sealant? Build a little angled cap to send water away while leaving room for it to breathe?
Here's the claim by the manufacturer, which is debatable:
Moisture is not the cause of decay but is one of (3) conditions needed for decay to occur. The conditions: (1) Moisture, (2) Oxygen & (3) Suitable temperature. These conditions enable the soil-dwelling or atmospheric decay fungi, as well as termites & other subterranean insects, to feed on the wood fiber. So the Decay/Degradation of ground-contact wood is not CAUSED by moisture but by the action of wood destroying organisms feeding on the wood. The 'chemical preservatives' protection strategy is to 'poison' the wood fiber, rendering it an unsuitable food source for the wood destroying organisms. The downside to that approach is that all chemical preservatives eventually lose resistance to decay and insect infestation. Post Protector becomes the perfect complement to the pressure treated (or cedar) post by simply providing a physical "barrier" isolating the post so the wood-destroying organisms have NO access to the post.
The assertion is that by eliminating contact between soil and wood, decay is prevented. Since posts set in the ground are always wet anyway (except in the desert), the claimed functionality is not affected by lack of drainage. In fact, drainage holes would seem contrary to this product's stated mission.
Since you've opted to use the product, I would do nothing.
From the installation guide for the linked product:
Optional Drainage Holes- On building will have posts subjected to the elements, (pavilion, open sided building, porch post, etc.) Drill out drainage hole dimples. These dimples are centered on all (4) sides of the base “horizontal” uplift rib. They allow drainage for rainwater running down the post and through the Post Protector. We suggest 4” – 6” of drainage stone for initial backfill material.
As the drainage holes are listed "optional" I would recommend doing nothing unless you want to dig it up and do it properly (you indicated you do not).
The danger is that water running down the face of the wood post and inside the Post Protector has nowhere to go, this is in contrast to direct-buried posts and concrete-enclosed posts where water can drain away. This may cause problems in the future.
The manufacturer includes a phone number to call for special custom installation guidance, you're likely to get a better answer from them than from DIY.se users who have never used this product: 877-966-8768