I'm sure I am not the only one who has experienced the "geyser" that occasionally comes up thru the Mirage Drive Well. Here is my solution. The material is called Dry Fast 50.
Dry Fast is a reticulated polyester foam, primarily used in making outdoor cushions. Its perfect for this application because it easily conforms to the space between the Drive Well Plug and the sides of the boat thus preventing the force of the water from pushing thru. It's open cell structure allows water to rapidly flow out, so it is not a tight seal. I found some remnant pieces at a local sail & marine canvas shop. I paid $20.00 for a 48" x 48" x 3/4" piece. (Enough to last me a long time). They called it "Dry Fast 50" (50 indicating the density). It cuts easily with just the blade from a box cutter and a straight edge.
The ones I made measure 15" x 3 ½". I attached a short line to one end, and tied the other end to the Crossbar so I don't lose them. I stuff it down into the back end only
of the Drive Well. I found that using just one on the back side was best, but I may experiment with an additional narrower piece (15" x 2") on the front end as well.
Another use: I cut a larger piece that just covers the front Drive Well Area where I lay my anchor so it doesn't scuff the boat.
This is a picture of what I call my "Old Man Rope".
It's a piece of 3/4" nylon? about 45" long with two figure eight knots that I use it to help pull myself up from, or down to my seat. Go ahead and laugh but it really helps, and at my age I'll take whatever help I can get.