Getting a 40 foot wide piece of plastic is way too expensive for me. I create a wider ice rink by hot glueing two sheets of plastic together. If you cut the 100' x 20' plastic in half you can get a 50' x 40' sheet of plastic when you glue them together. The rink will be smaller than this because you need the plastic to go all the way over the boards (do leave it too small, as it's a big problem if the plastic won't reach the frame.) You could easily do a 36'x 44' rink this way if you're careful about making it square.
Here's how I glue them together. It's best to have two people do this. One to work the glue gun and the other to press the plastic together.
First, cut the plastic in half. Next, place the pieces next to each other where you've built your rink (It's best to already have the frame done at this point.) Overlap the plastic by about 1 foot. Have one person hold the top plastic piece back while the other makes a serpentine glue strip with the glue gun. Be sure to get lots of glue on the plastic. (I've even used 3 people while doing this using two glue guns. You want to make sure there won't be any leaks.) As the glue is going on have the person holding the plastic press the top piece onto the bottom. A rolling pin works really well for this, as it squishes the glue and makes for a wide joint. Do this all the way down the joint. If you have any week spots in your joint put some more glue on them. You can even cut a piece of plastic from the edge and apply it as a patch, if necessary.
When you're done, staple gun the plastic to the outside of your frame, leaving plenty of loose plastic inside the frame. The water will push the plastic all the way to the edges of the frame. You'll want to start filling your rink as soon as your done gluing. It takes a while (overnight?) to fill a rink of this size unless your ground is very level.
Once the water is over your joint you should inspect it. Just look in the joint for where the grass seems darker underneath. This usually means you have a leak. If this is the case, I've found waterproof tape or extra heavy duty duct tape sometimes works. Another good solution is a zip lock bag filled with sand. Just put the bag over the leak to slow/stop it. If you have a minor leak you may not have to worry about it. I've had leaks that were slow enough that the water was able to freeze before I really lost too much. Once the ice is 3 or 4 inches this it doesn't really matter if you have a leak, as you can build up the ice from the top.
I'll try to update the site with this info and maybe remember some further details, so check back in a few days.
BTW, I purchased a 100' x 40' sheet of plastic two years ago for about $300. Fortunately I was able to use it for two seasons, so the price wasn't that much more in the long run. If you can afford this, it's way easier (although this sheet of plastic is really heavy and quite a pain to store.)
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