There are several methods people use to make sure their slabs don’t become too long or wide as they muffin under vac. Some put chopsticks or something similar under the edges of the silicon mat or parchment underneath their material to create a small lip, which works pretty well for oil that has already been purged to some degree inside the extractor.
Others, like Derek Cumings and the team at Incredible Extracts, use a tray resembling an inverted shoe box lid made of folded parchment paper. This method is particularly handy for those who prefer to pour their material from their extractors rather than scrape material that has already been under some level of vacuum.
Here’s how it’s done:
1) Cut your parchment to size — figure out how long and wide you want your tray to be and how high you want your walls to be. Double the desired wall height and add that to the desired length and width of your tray to determine the size of your parchment. For example, if you want a 10 x 14 inch tray with 1.5 inch walls, cut a 13 x 17 inch piece of parchment. Make sure to keep your walls low enough that you can still watch your material as you pull vac so you can maintain control of your muffins.
2) Fold your walls — make folds along the long edge of the paper. In this example, fold a 1.5 strip at the top and bottom of the paper, then fold a 1.5 inch strip along both ends. Try to keep all your folds straight, even and consistent (use a ruler or straight edge for best results). When you’re done, you should see a perfect square in each corner of the paper.
3) Build the walls — stand the walls up and bring them together at the corners. This will create a triangular flap that sticks out from each corner. Crease the “bottom” of the triangle, which runs up diagonally from the bottom of the tray.
4) Tuck the flap — fold the triangular flap so it sits alongside one of the walls and paper clip or staple it in place. Do the same thing to the other three corners and and you’re done.
Note: make sure there’s some room between your parchment and the sides and back of your oven and that you aren’t blocking any inlet or exhaust holes.