Plug the ends of the brass tube, then mount the assembly inside a (plastic) tube leaving top end open to receive the casting resin.
The outer tube can really be just about anything. . A drugstore prescription bottle of appropriate size can be used and is good because it already has a bottom. . When the casting has cured, you saw off the bottom, remove the plugs in the brass inner tube, turn off the outer tube.
You really need to give more details about the desired size of your blank if you want to get more specific suggestions. . Also the resin.
Also, have you considered that the seeds might eventually deteriorate ... wrinkle and rot ?