Can someone tell me the best way to put a transparent "window" on an opaque plastic bottle, while retaining the bottle's ability to contain liquid without leaking?

Context: I am using a composting toilet (ok. it's a bucket) with a diverter for urine. Right now, I'm using a clear bottle. It isn't always easy to tell when it's nearly full, but it's possible.

I want to trade it out for a detergent bottle because of the drip-free spout. Its opacity is also a feature, so that no one has to see how gross it gets when I go outside to empty. It's also larger, which would reduce frequency of emptying.

The trouble is that there would be no way of knowing how full it is. I could buy an electronic scale to permanently set it on, but that solution seems overkill.

The simplest thing would be to have a window of transparent plastic near the top, so that I can see it's getting close, well before it overflows.

(Overflow is a realistic issue, and I know from experience how annoying it is to solve when it passes that point.)

I know how to cut holes in plastic. Easy.

But how could I place a layer of clear plastic (cut from another bottle) over the hole and seal it on well enough to be waterproof? Is this possible with the average tools I might have on hand?

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