I have recently learned how easily HDPE grocery bags can be "welded" together to make thicker pieces of plastic and that by "welding" together multiple thick pieces, just about any shape can be created fairly easily. In a nutshell, the most basic way to try this is to take a couple clean plastic grocery bags, and cut them so there are no seams - lay about 4-8 of these pieces atop each other and lined up as cleanly as possible... Next, place this square/strip/triangle/whatever shape inside a manilla folder or some other card-stock type paper. Make sure the plastic does not go out of the paper covering each side. Next, iron the manilla folder for 10-15 seconds over each area where there is plastic below with your standard clothing iron on a medium heat. I pressed down fairly firmly to ensure no air bubles were trapped. When you are content with how much you have ironed the plastic, let it cool (slowly while pressed between a couple phone books if you want it very flat), then take it out of the manilla folder - vioala! you have a square/strip/triangle/whatever of thick, durable HDPE plastic! If you need thicker material, you can "weld" multiple thick sheets together or start with more sheets. If you need a larger sheet, overlap many smaller sheets with about 1" of an overlap, and "weld" them together... I am just finishing making a slide for my toddler for him to use inside... I used wood from a couple pallets to make the general shape, and am covering the entire thing in 12 sheet-thick HDPE made from shopping bags. I have made potting containers for plants in a large variety of shapes and sizes, even tools for use around the home. After making these sheets, the heat from a blow-dryer (a heat gun is more efficient) is enough to make it maleable so it can be formed into virtually any shape! There are some things to remember about HDPE when making things, some of which are arguably its most appealing and its worst charachteristics. HDPE is chemically inert. This means that it will not interract with any chemicals it comes in contact with. This includes ink (yes, I know grocery bags have print on them - a bit of chemistry involved in that discussion), paint, glue, solvents, epoxy, or any other chemicals or anything else. On a good note, this means it will not degrade into its contents (which is why it is used for foods), and the contents will not degrade the HDPE either. That means it can contain virtually any chemical with chances of breaches being minimal. On that note, being chemically inert also means it will not naturally bio-degrade (and on the flip side, it will not decompose in landfills into something harmfu)l. Re-utilization and recycling of HDPE is the best practice for the environment.