I use an expansion tank for my fresh water and for my webasto system. I paid less than $25 for each of them on ebay. Hard to build, convert or modify something for that little $.
Does the expansion tank you used have an inlet and outlet?
I really would like something that can do both expansion tank and buffer tank. Due to time constraints it is likely I will just buy an expansion tank with radiator cap from Sure Marine for about $150. Yes, I could save $100 by doing it myself, but I just don't have the time to procure something and get extra nipples and such welded on.