I know that the LFS had some smaller pieces mounted on slate plates to hold them down. I have no idea how much weight it would actually take to weigh it down, but more than I have available

I'll be calling the store tomorrow for their suggestions. If the 5lb+ lace rock I put on it won't hold it down, a plate of slate sure as heck isn't going to
Unfortunately, I have nothing large/deep enough to keep it fully submerged in - even with the overflow blocked with a washcloth and the tub filled, it won't fully go under, even held down.
<sigh>
I put the smaller chunks I had to cut off in the tank and will see how long they take to sink.