Ordered my hdpe off Amazon and it cuts like wood. I would recommend 1/2 inch as the 1/4 seems to not want to lay completely flat at the edges.
Did you silicone it to the bottom?
Once you silicone, it should be fine. Plus it’s recommended to silicone so detritus and other undesirables do not make their way under the board to rot.
If you silicone, report back and let us know how it adheres to the HDPE. Some of these materials apparently don’t glue or silicone well. It doesn’t stick.