My sub box for two 10w7s is built out of marine plywood and fiberglassed. It's heavy, but it sounds great; and it's lasted for years.

MDF in a marine environment is a bad idea. I know the properties are great for building driver enclosures but I would gladly take the trade off of a water resistant material. Just resin is not enough to seal MDF. It will crack and break down over time.

I know MDF is not good to use in a marine environment and I would not use it for anything besides a sub box. But I look at a sub box as something removable that may need replaced every once in awhile. I'm a sound guy so I would rather have the better sound. Most people would not know the difference in sound between a mdf and coosa or some other composite boards. So I'm sure coosa would work fine, and use the higher density stuff. I would still not use lexan/plexi. If weight is an issue then just find somewhere that you can cut a hole in a panel that will reasonably separate the front wave of the sub and the back wave and mount it with no box. It will player lower and almost as loud.

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I built this box out of MDF then premiered and painted. It stays outside in the weather all year long and has taken a lot of abuse. It was several years old in the pics with the girls dancing on it.

Forget MDF go to Lowes and buy a sheet of Advantek It's way more dense and near water proof. I've had a sheet in the bed of my 1 Ton since 2009 still 3/4" thick and solid as the day I put it there.

I give up on MDF a few years ago they don't make it the same as 10 or more years ago seems like pressed paper anymore

I was talking with a composite materials distributor and he mentioned expanded PVC sheets. We It was my intention to use it for transom core at the time but it may work excellent for a box. It was explained to me that it was high density. Maybe glassdave can confirm this.

Lexan works just fine for sub boxes. It used to be the thing back in the day. It was just very expensive. I would look for HDF and resin the heck out of it myself, or use marine ply and resin it good.

If you look at my build in the cig section you will see 2" think foam board glassed into the gunnel that's the speaker boxes. The foam is the same stuff used in ice boxes on shrimp boats. No rot and very dense. It's also light. All you have to do is resin it. Will try and get more pics
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looks like its normally used as subflooring

Ok guys, so we looked into the options, and we found in our situation it would still be best for us to use MDF. I looked for the Advantek and coosa, but neither was very readily available, and truthfully, I do not mind replacing it every few years. So, I now have the next question. What type of resin to use. West systems epoxy or west marine polyester resin. Is hardener necessary? Also, would it be beneficial to put a layer of fiberglass on it, or would that just be a waste?

Thanks for all the input.