What about this. i might get 6 of these.
Generally, the faster algae grows upon the structure, the better it will hold crappie.
I would use that as a base. Stick branches inside and secure with zip ties. Weight the bottom and deep six it. If you want to make sure it stands up, hang off a dock, or, secure rebar legs at the bottom to make a wider base and slowly lower into position. You should be able to make a dandy brushpile with that.