We are getting ready to dig a second pond this winter, currently drawing plans, scratching them on the ground and then doing it again, hoping to get something we really like soon. To help protect our beloved Koi from predators and the blazing hot southwest sun I am planning on including two good sized (approx. 3'x3'x2') cave areas. I plan on the top of the caves being at least 3' deep. I know for a fact that we have mountain lions, bobcats, skunks and coyotes in the area, we are the rural wild southwest. I'm sure there are others in the area that I just have not seen, we have lived in this area for 25 plus years. Can anyone give any suggestions as to what I can use to make the lids for these areas, I was thinking of possible hardy backer board like you would use for installing tile. It is a concrete product and I have been wondering if it would letch anything harmful into the water. The last couple of years we have been experimenting with bog tubs and have found them to greatly increase our water quality. Would these possible take out anything that might be leatched into the water. The total gallons will be 5-6ooo and 5'deep.Thanks for your help.