I don't know how many exist anymore, but Pro (I believe) made a "landing pad" for the gen 2 battery hanger. It mounted around the angled battery allowing the sled to safely sit on the ground. I believe it could be modified to hold a weight without obstructing the battery.
Someone else here who's been around longer might still have one or know someone who does as well as confirm or deny my statement.
Alternatively, Greg Bubb (XCS Inc) and Alan Rencher (Rencher Industries), both make a gold mount plate that you could then screw or mount weights to. This assumes that you're not using one of the battery plates.
The only other thing that comes to mind is having someone make you a couple small plates that are drilled and tapped for little dive weights that you can mount onto the sides of the lower electronics (I believe the lower electronics have mounting points for accessories on either side). That keeps the weight centered at the post and you can add them equally on either side to get the desired additional weight.