Well, they need a new name tho.
I'd call them "concrete weights" cause nothing short of a mammoth is gonna "drag" that thing anywhere, lol!
I think this is a pretty good idea tho. I do have a few suggestions:
Beaver: You can scout out your beaver locations ahead of season and go make the weights on-location. Make it a cylinder tho. The reason being for the cylindrical shape is for the ease of getting it out of the ground. (You can use old tin stove pipe for the mold maybe?) Pour it near the bank of the river/stream/pond. Come back later when it's set. Dig around the cylinder, making a sort of ramp for it, and roll it into deep water. Rolling will be much easier than picking it up.

I would suggest dying the thing so it doesn't attract human attention. For one, humans will think of concrete in the middle of the woods as a sort of littering. Also, you don't want hikers/hunters/other outdoorsy types investigating your block, trying to figure it out. Make it brown so it's just like a log.
Unless you leave it out in the woods/fields for weeks, canines probably won't want to investigate it. They usually hate new things that suddenly appear in their environment. Some of the trappers on here have called it "neophobia," fear of new things. If it seems more natural, they might adjust to it better. Perhaps cover it up with divets of grass? I'm no expert so you might want to consult some K-9 men about that one.