Try to determine how much weight you will need to make the dome neutral in the water because you may not have enough room for the weights. Do not put the weights on the inside of the shade as they may show up in your picture. The other problem you have with that solution is they are not removable for transport, Over under pictures etc.
I have the same problem with a prototype 8.25" hemispherical dome. It is 3 lbs positive. What i did is drill holes in the dome shade, threaded the holes and attached two Velcro cable ties from Micro Center. It works best if the weight can be placed near the front of the dome as opposed to the back-plate. Please look ate the attached pictures and let me know if you have any questions. It is nice that the weight is removable for travel and it is a standard scuba weight. It could be a soft weight also in that was all that was available.