Use polyurethane sealant with Plexiglas

I have a below-grade basement window in a lightwell. In heavy rain, the lightwell fills to the bottom of the window and then the water leaks into the basement. I've been through a lot of options and my current plan (plan D) is to screw a clear plexiglass sheet to the window frame to physically block the water but still let light in.

Question: is it OK run a bead of polyurethane sealant (e.g., Sikaflex) along the edge of the sill before I screw the plastic to it, as a waterproof barrier between the wood and the acrylic plastic? Another question in this forum said you should not use silicone because that contains xylene, which is bad for the plexiglass. In net research, I've found any number of answers to my question, all different. Mostly that it won't stick, or I need some special-order primer before using the sealant. Considering that the screws will hold the plastic in place and against the wood, will just the polyurethane provide a reliable waterproof seal, especially between where the screws are?

In reply to comments questioning why I am attempting this: I've tried other things.
1) I thought the lightwell was filling from direct rain. I built an awning for it that also made sure water wasn't wind-blown in from the sides. Didn't work.
2) Turns out the light well consists of concrete walls around a mud floor. When it rains hard, the ground saturates, the water level outside is higher than inside, and the light well starts to fill. The bottom of the windows is still several inches below ground level. I looked into adding a concrete floor. That ran into several problems including not being able to secure the concrete walls firmly to the existing foundation, and creating a "boat" that will want to rise up with the water level.
3) French drains won't work–remember we are below ground level, there is no place for the water to drain to. The contractor who suggested one wanted $12k to put it in, and remained vague where the other end would be.
4) I could install a sump pump outside and pump the light well to some undisclosed location. This is getting into a bigger project that I was hoping for. I could pump it a long way across some landscaping and a driveway onto the lawn. Or I could try to pump up into the kitchen line 15-20' away that flows into the sewer.

It only rains hard enough to spill into the basement half a dozen times a year. As far as I can tell, the previous owner cleaned and painted the concrete in the basement, but this must have been going on for decades and the window frame hasn't rotted (yet). Its only the bottom few inches of the frame we are talking about. Solving the problem seems difficult and expensive–I was hoping the brute force hack would do the job, with annual vigilance.