Anhydrous acetone will probably be more convenient than ice cold MEK because cold objects condense water from the air and if the MEK gets wet it'll dissolve more of your mescaline.
When I 'washed' mine in anh. acetone what I did was to grind it to a powder [a slightly sticky powder], pour the anh. acetone over it in a beaker with a stir bar, covered the beaker with aluminum foil, magnetic stirred it for 30 minutes, and filtered. This yielded an off-white mescaline hydrochloride in 90% yield and a dark solvent that evaporated to nearly black goo thats still sticky months later. I then recrystallized my off-white mescaline•HCl in anhydrous isopropyl alcohol getting only 72% yield of a snow white mescaline•HCl and orange solvent that evaporated to yield pumpkin colored crystals. I havent assayed the pumpkin crystals but I cant honestly tell any difference between the off-white and the snow-white mescaline so in the future I will probably only do the anh. acetone wash unless I want to impress friends and seduce women with my uber ultra-clean mescaline
If you do not have a magnetic stirrer and dont intend to build one I have seen people do the process the same way but using a spoon to mix it, sort of grinding the mescaline mush against the wall of a glass bowl for several minutes. Their product wasnt as clean as my off-white stuff but it was always a very clear improvement.
Here are pictures of my black goo and pumpkin mescaline byproducts.