Brandi, your water looks pretty clear. How are your fish doing?
That endcap looks pretty thick. Do you think you could drill some small holes around the perimeter into the endcap and then put some #6 stainless screws into it to secure it into place? I'm not sure how you plan to do your zipties, but keeping that endcap in place and water-tight is going to be the biggest challenge. When water rises around the pipe before it overflows, you are going to have a very powerful lift of that whole end section. It's like a two-place kayak. You might have to put an anchor on the end of the pipe to keep if from flexing upward. It probably won't flex too far, but over time, the up/down flexing with water level will surely be an issue. How big an issue, I'm not sure, but it certainly will tend to float upwards.