I've done this before, experimenting with a few different combinations of rosin scraps. Made some nice messes, but finally had good luck melting it (gently) in a cup made from aluminum foil over a little alcohol lamp, then cooling in the fridge. Before it gets really hard I peel off the foil, re-warm a bit and wrap little bricks in parchment paper (like you use for baking - it's silicone coated paper so it peels off cleanly). Works great. And why rosin always comes in round cakes I can't figure out; bricks work a lot better!
