The opening only needs to be large enough to accommodate the diameter of the wick. It might be of some benefit to you if we started a dialog about your o/a set-up. That you DO need a wick is a pretty clear indication you could probably do better by adopting a soil with better drainage/aeration.
Generally, those that use coir commercially limit it's presence in container substrates to 10%, 15% at the max. It's fine particle size (promotes excess water retention) and extremely high K (potassium) content can both be problematic. Often too, it's very high in soluble salts, perhaps as a result of having been exposed to seawater during processing. It's difficult to try to nail down a specific cause related to your problem because there are several potential possibilities, given the high volume of coir you're using.
If nematodes aren't an issue in your area, partially burying the container might be helpful until the planting has matured to the point where it needs watering at least every other day while still partially buried. This employs the earth as a wick and turns your pot into a mini raised bed, hydrologically speaking. It should resolve the 'drowning' issue if in fact that is the source of the trouble.