Inno,
> it is my impression that Realtraps are not just a stack of 703 fiberglass in a frame. <
Correct, they are not just a stack of 703 rigid fiberglass.
> a thick panel of 703 absorbs all frequencies and I think that realtraps do too. <
Not so. You can see the absorption characteristics of all our traps on the Product Data page of our web site.
That said, a 4-inch stack of 703 will do a fine job. When someone has more ambition than cash, it's a great way to get decent bass trapping for not a lot of money.
--Ethan