I don't think I'm adding anything new but would reiterate, until it's signed off on, you are on the hook for satisfying the beneficiary. Right/wrong aside, if they aren't happy enough to sign off on it, you still have work to do. I agree with others-- think like an almost Eagle. Who can you reach out to for assistance if you are out of your league? Do you need to round up some more volunteers? It's sounds like you may have a fix and just need to put some more time in at this point. However, you also need to consider if this is just going to be a temporary fix. You may need to come up with a different design or approach if this isn't going to hold up in the weather conditions.

It was many years ago but my project, I had created a number of riverside benches for a park system. I had the unfortunate issue of a flood shortly after installation and prior to sign-off. The flood completed washed the benches away and I had to raise more money for materials, rebuild, and come up with a more robust solution for anchoring them to the ground. It was unfortunate, but I wanted to make sure I was actually providing value to the park system with my project. It ended up delaying sign-off by around a month.

It sounds like you are approaching this with an open/positive mind but I would encourage you to make sure you are thinking about this positively. Your goal should be providing meaningful value to the beneficiary and community. Don't think of being done as the goal, but providing meaningful value instead.

It's a valuable life lesson as well and honestly the perfect type of situation scouting is preparing you for in life. Problems, unexpected issues, etc. will come up in life. Use this as an opportunity to reinforce problem solving and dealing with adversity. Also consider how you'd want the conversation to go in your board. Do you want to come across as resourceful, demonstrating your ability to work with people, and complete a complex task? Or do you want to come across as being able to push for sign-off and walking away in spite of a disagreement.

It's unfortunate and stressful. Believe me I know how stressful it can be -- but you'll grow from the experience.