ABS is a pretty hard plastic. Why would you need fiber-glass in the mix? Fiber-glass is for structural strength, by that I mean holding up a 20lb sub-woofer. You're making holder for your .125lb phone. Fiber-glass is needlessly complicated.
The sludge would work only if you have enough solvent to make the sludge viscous enough to saturate the FB. Do a test piece and let us know.