What we need is a layer of fabric to absorb sweat and to reduce the amount of sweat generated. The best fabric for the task that I have found is cotton terry cloth. Choose a color close to your breast form color. They make towels from it. If you have seamstress skills, make a pad from a piece of it, stitch up the edges to prevent fraying, and slip it between your chest and the breast form. If I you lack seamstress skills, find a wash cloth, fold it into a triangle, fold is down again until its size covers nearly the same area as the back of the breast form, and slip it between your chest and the breast form. It may slip around a little but not a lot.

Using adhesive behind a breast form to hold it in place can become a sweaty, sticky mess. Glueing breast form to bra, not to chest, holds things in place and is not nearly as messy.

The management of the storage facility where I dress claims to regulate heat and cool in their storage units. Before a recent change in management, temperatures sometimes were as high as 60C, too hot to even enter. The new management keeps things below 30C. Outdoors, the warmest that I have been dressed is 30C. My problem is more often cold temperatures. The old management at my storage unit routinely let temperatures fall to nearly 0C but the new management keeps the low end near 15C. No sweating at those low end temperatures. I warm my breast forms on my thigh for a few seconds, less sensitive to cold, before inserting on my chest. Breast forms warm quickly to near body temperature after insertion even when I use the same terry cloth buffer.