You do excrete 50% of methamphetamine unmetabolized in urine. If I were going to try it I would start by bulk neutralization with a strong base like NaOH. I would go until I saw oil floating on top, which would be the freebase meth. Ph 12 should do it. I would add to the oil that I probably pipetted off the top 3x the amount of naptha or another non-polar solvent and reflux it for 30 minutes. Then I would add 1/2 the amount of distilled water 3x and after I shook it for 5 minutes, but before I separated the polar phase (the water) I would check and adjust the ph back to 5. Then I would bubble hcl gas though it until no more was precipitating and run it through a butcher funnel and rinse the powder with ice cold acetone until it was stark white. Then I would recrystallize it in ethanol/ acetone