Step 3: Building the Ice Rig
You'll want to setup the saw-horses (or whatever you're using for supports) as close to the snow base as you can get, to minimize the distance you'll have to move the ice top once it's made (trust me, this sucker's heavy AND fragile). Lay the MDF boards on top of the saw-horses and break out your level. You need to have these boards be as level as possible, so if the ground you're building on is not flat, you need to make adjustments to make sure that the boards are laying flat. You can't really see in any of the pictures I've provided, but there are smaller, thinner boards wedged underneath the MDF and the saw-horses that buttress the left side of the table up to make up for the uneven ground.
Allow me to deviate here just for a bit to relay my experience with this part. I built the ice top using only one MDF board (I can't remember the thickness of it now, but it wasn't terribly thin), and unlike what happened with the guys at Wisegeek, my ice expanded down as well as up, particularly around the corners. This is not good, as it creates an unlevel surface on the bottom of the ice sheet. Now, since the base for the table is made of compacted snow, it doesn't affect how straight the ice sheet sits on the table as much as you might think, but it does add unneccessary strain on the snow basin structure, particularly on the corners. So, I reasoned that if you used two boards and stack them on top of one another, then would be enough reinforcement to guide the ice more upwards than downwards; plus, having two boards instead of one helps with constructing the snow basin, as I had to make due with just one.
Once you get it even, lay the beams around the perimeter of the MDF boards (such that they stand 4" tall); if you've measured right when you bought/cut them, they should form a perfect rectangle that sits on the edge of the MDF. Secure them in place with something temporary (I used Gorilla tape, which SUCKS in cold weather so DON'T USE IT! If you have large C-clamps, those would work the best; put two around each corner and maybe one in the center of each beam and you should be golden). The beams themselves will not be subjected to that much pressure; they're there more as guides for the ice.
After the beams are in place, line the whole inner part with the plastic sheet. If at all possible, try to smooth out the bottom as much as you can, as it will make it easier to detach from the ice block later. (In fact, I was never able to detach mine, so in the end I just trimmed the excess plastic away rather than try to separate it; since it's clear, its invisible in the final product). Secure the plastic to the beams either on the top or outside of the beam, not the inside. Then place the whatever weights your using on the corners to minimize the edge of the plastic from flipping back into the water basin (see the photo for what I mean by this).