Cartridge too heavy for tonearm counterweight...

I bought an Origin Live-modified RB 250 tonearm.
I'm mounting a Clearaudio Veritas on the tonearm for a friend, but we found that there isn't enough weight with the stock counterweight.
Any suggetions as to what I can use to make the weight heavier? I had thought of Blu-Tack, but not having done this before, perhaps that's not a good idea.
What would you use? And please, this is the ONLY cartridge that the owner wants on the tonearm as he paid $400 for it, so another cartridge isn't an option...YET.
If your friend doesn't mind spending a few more dollars, I strongly recommend he replace the current counterweight with Express Audio's Heavy Counterweight. Not only does it weigh more (which will solve your need for a heavier counterweight), but it is a much better weight than the stock unit. Many of us who bought the Express Audio counterweight have been pleased -- even somewhat amazed -- at the improvement to the Rega tonearm's performance. I wrote a very positive review of this counterweight several years after adding it to my Rega RB-900 arm. You can buy the Express Audio counterweights (both the standard model, and the heavier unit) from Audio Advisor:
I had a similar amazing experience with the the F-2 Titanium Heavyweight Counterweight sold by Gregory_Kerry, a member of Audiogon. You can contact him through the internal Audiogon member email contact system. I put it on my humble Rega RB-300.
This single upgrade astounded all listeners with the enhanced solidity, bass weight, and bass extension, along with clarity, harmonic richness, and musicality (overused word, but really appropriate here) over the entire audible range. You can tell I'm a fan. A great item at a fair price is deserving of praise.
It seemed to mate beautifully with my Non-felt record mat, although someday I have to see if Herbie's turntable mat is a further upgrade, as its website promises.
It is a great idea for all of us budget-minded, vinyl loving audiophiles. The current price is around $130, and well worth it.
Origin live suggest a cheap fix is to superglue metal washers to the existing counterweight. If you want to try it out first you could use a rubber solution glue, like goop, and then if it doesn't work at least it will be easy to get the washers off again.
It should have mentioned that a friend bought me the Express Audio Heavyweight. Unfortunately, it doesn't fit on the new end stub of the modified Origin Live tonearm. He tried it today, and it was too small by 1/64 of an inch.
So, now I own a new counterweight that won't fit the tonearm. A bit of an annoyance, to be sure.
When you buy a counterweight for an OL arm or an OL modded Rega arm, you must specify that it is an OL end-stub on the arm. Otherwise, you already know what happens.

If you must add weight to the counterweight, add it at the bottom. This will help the tracking and the sound.
Try bubble gum. Sweet midrange.
Use 2 sided carpet tape to adhere a round washer around the center hole. Both sides if needed.