Is there an inexpensive alternative to the marine 2-part expanding foam? I was looking into the expanding foam from hardware stores in a can. Will any of them work?

You need closed cell foam. Most cheap foam is open cell and it will hold water like a sponge. Plus marine foam is pourable and has a higher density. I would not use off the shelf stuff. Maybe a spray foam insulation contractor can help you.

Call us composits or look them up online. It's not really that expensive. I think 80 bucks or so for a gallon of each part. I had plenty to foam in a 60 gallon tank and the stuff is rock hard when cured.

Ok ill look into them. Thanks