The filler i have used for a LONG time with foam plugs is a mix of DAP ultra light spackle and Durham's Rock Hard Water Putty. Both are available at big box stores. I get mine from the Lowe's next door to my office. The light spackle is the one that when you pick up the tub from the shelf, it feels empty.
I use a mix of about 70/30 by volume, spackle to Durham's.
Tom