Paul,
What part of the country do you live in and where have you looked for expanded PVC? Also, what did you call it when searching? I have found that a lot of places refer to all expanded PVC products as Sintra, which is historically the most common brand name. Sort of like asking for Plexiglas when you really just need any old acrylic.
I don't know of any websites that show how to work with the stuff, but it is like working with plywood for the most part.
There are a few issues that make this stuff more difficult to work with than ply, however.
Unlike plywood, mdf, melamine, etc. most of the sheet plastic I have seen does not have a square and straight edge to work off of. The edges seem to have a natural bevel to them and the corners are far from square. This makes for a lot more work when cutting sheets. I don't claim to have seen multipe samples of every expanded PVC plastic on the market but I have been to plastic distributors before and noticed that this happens a lot with plastic.
Joining is a bit more difficult as you need much better junctions between two sheets if you want to solvent weld. It is not quite as forgiving as plywood, melamine or mdf. And then most people only work with 1/4" or 1/2" so you don't have quite as many choices of fasteners, although it does take screws fairly well. I really like to use 1/8" aluminum rivets for attaching PVC panels together, although 1/4" PVCX is a bit thin for this. And on that note, I do think that 3/8" or 10 mm PVCX offers a nice compromise between weight, cost and ease of construction. Just thick enough for screws or rivets but cheaper and lighter than 1/2". It is not a common size, however.
I just looked up the ad in the classifieds and the prices listed are very good. Shipping could be a deal breaker, however. One issue I want to point out here when looking for PVCX is that there is quite a bit of variation in weight among the different products.
Just looking at 1/2" PVCX, for example, you can find products that weigh significantly less than 1/2" plywood to more than 1/2" plywood. Shop carefully if weight savings is one of your primary reasons for being interested in PVCX.
Lastly, look for a distributor that will CNC mill the sheets for you. By CNC milling every piece will come perfectly square and the shipping costs will be reduced.
Let me know where you live and I might be able to recommend somebody.
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Currently keeping a small collection of various Gonyosoma. Both G. janseni and G. oxycephala.