We used microballons mixed with coarser sawdust in our yak, and in a series of pours. The first was tilting the yak so that the bow was angled down, pouring a bit of epoxy into the vee of the bow, smearing it around a bit, then spooning in a thick, "stiff" mix of microballons and epoxy. The idea was to provide as light of weight filler as possible, and to fill in that tight inner vee shape. Next day another spoon-feeding session only angling the bow down at an even steeper angle (I'm omitting the Rube Goldberg contraption that hung from the rafters to keep the yak in position that only took a few million miles of nylon rope). Anyway...the idea of the multiple "pours" was to place the stiff mix into and fashion it where I wanted it to go, and to reduce the HEAT. The microballons act as an insulator, and the temp goes up remarkably fast, hence the layers (too much epoxy in one spot with filler mixed in will generate temperatures that give credence to the China-Syndrome idea). So after joining deck and hull, the final pour is minimal in volume, which is important in that the mix must be more epoxy then filler in order to pour, which means a denser goo, a hence, more weight. Dean had a good idea using a soft wood, carving it to shape, and epoxying it in place. The idea is so good that I'll just have to try my hand at making a stitch-and-glue next, and seeing exactly how light a yak can get...