I have done a little research on old pincushions. I've found in the old days horse hair was used a lot. I asked my husband to collect some for me when he trimmed his horses mane. I used shampoo and dried and aired outdoors then stuffed a pin cushion for a gift to a friend. I have found a place online to order metal filings. These came with a swatch of fabric somewhat like swim suite fabric to hold the filings. After that use the fabric of your choice. This worked well for me and the filings keeps the pins/needles sharp. A fabric like swim suite would also hold the sand without leaking also I would imagine.