On reconsidering my reply, I really did not read your post correctly. Since you want something waterproof, that would not be landscape felt (or newspaper/cardboard, as [email protected]
suggests). You pond liner idea would obviously be non toxic. However, anything if that type would get torn over time due to digging and cultivation. Aluminum flashing would be a pain to install and you would risk injury to you hands on the sharp edge. (Also it will be expensive and I would think you might as well buy ready made raised beds.) The main problem is that whatever you use is going to trap moisture between it and the wood which will be even worst than no barrier at all - and encourage mold due to the exclusion of air that you find in soil. The moisture will get in there whatever you do. You would need some form of rigid liner that provides an air gap - in which case the cedar would be basically just cosmetic. Our raised beds are made of regular untreated 2x10 lumber. They have been working just fine for 8 years now, and are showing no signs of disintegrating. Cedar will last a lot longer than untreated lumber. At our last house there was a small lake, controlled by a weir box about 18 inches wide and 3 ft deep. The weir dam itself was made of Cedar boards less than 2 inches thick, installed in 1946 and constantly underwater on the up stream side, partially on the downstream side. It was still holding water when we left there 8 years ago. 65 years or so isn't bad!! You shouldn't have any problems with just good cedar boards for your raised beds.
The reason liners are used in Cedar patio planters, it to prevent soil and mater from leaking through the loose joints they use for those decorative items and staining them and the patio.
Incidentally, the reason I lined the bottom (only) of our beds with landscape felt was because they are installed on top of a gravel area and I wanted to stop the soil and gravel mixing.