I have a small concrete pond about 4 ft x 8ft and about 3 ft deep. I mainly use it is a water reservoir for my garden and have some plants in their too. And of course a small waterfall and some rocks to keep it interesting.

For safety purposes I am thinking of putting a submerged shelf/deck (I do not what else to call it) that is about 18 inches high and covers the entire pond and will be submerged most of the time. This will effectively reduce the depth to 18 inches but still hold twice as much water.

The shelf needs to be strong so that at least one adult can stand on it, it should be light-weight and removable for cleaning purposes.

I was thinking of getting hold of those FRP gratings they use commercially and cut them to size and put support posts of suitable material. I am not sure whether it is a suitable material, is it a good idea and where to obtain it. They seem expensive and does not look like the companies would sell 30 sq ft or so.

Any ideas or alternatives will be very helpful for me. Thanks.