Got a tip from my local hobby shop, and it works great! Once you are about half-way done with your Tamiya styrene cement, cut up a bunch of white styrene sprue. Throw the little pieces into your glue bottle and wait overnight. The plastic will resolve into the cement. Keep adding sprue until you get the desired consistency and viscosity. I use this to fill cracks now and also glue pieces that lack surface area (it will fill corners like a filet or radius). Works great!