With the Monsoon and Phantom you could get the bulb to be anywhere between 1600-1800 gm using lead shot. You could get to 2000gm cutting the bulb open and laying in lead sheet.
The Focus bulb is 2000gm. Just looked on HWH site and they want $65 plus delivery. That’s 1/5 of the price of a new one including a radio. Bit strong.
My Phoenix bulb weighs 995gm so could be a different version to yours.
I think 3000gm is going to be to much.
Have you tried just wrapping some lead sheet around the bottom of fin to see what weight you should be aiming at. Not very stream line but will give you a good clue to the weight you should be aiming for.
Maybe you can borrow a Monsoon/Phantom/Focus bulb and temp fix it to the your fin. Make sure it is secure as finding a replacement will be a big problem. Obviously you cant modify the borrowed bulb.
A Monsoon bulb at 1600gm would be the heaviest I would go I would say. The lead sheet on the fin would be my first try. Has been done many times before.