Breast prostheses

My mum had a lumpectomy at the end of Dec but then a follow up mammogram showed a second lump in the same breast so had to have a mastectomy just 4 weeks later. She is 76 and these 2 operations back to back have really taken it out of her. She is now waiting to start radiotherapy in a few weeks time. 

She is really upbeat in spite of this and is trying to find a well fitting bra to put her prosthesis in. At the minute she just has the soft one but is having problems with it and as she has quite a bit of seroma around her scar it will be a while before she can be fitted for a more permanent prosthesis. The problem is that she normally wore a 42E and her remaining breast is very heavy so the soft prosthesis is much too light and means she can't get her bust to look even and level. Can anyone suggest something we can buy or use to weigh the softie down a bit until we get a silicone one. We've tried it in both specially designed bras as well as in her ordinary firm control ones. 

Thanks

Star