To use as a lift bag, it should have a way to dump air so that you can control the buoyancy and prevent runaway ascents. An exhaust valve is typical. Also, the dump should be high up to be effective- all of the SMBs that I have place the dump at the bottom, which is great for deflating it at the surface, but is ineffective for dumping air underwater.
Also, a way to securely attach things to it would be useful too! If it's an SMB, it might have a little ring or loop at the bottom. Be sure it is sufficient to support whatever you attach there. It could be bad if the object fell off while you were midwater.
Lastly, these are some of the things that are covered in a Search and Recovery class or an Advanced OW class or equivalent. Good luck!
EDIT- I understand the idea of the lead slates, but it does seem like extra weight to lug around- if you are at the surface, you can hold it upright, and if you are using it as a flag/float/marker, you can keep a little tension in the line to make it stand proud.
