You have some options with foam. For cases you can look for a place that sells eggshell foam if you're not cutting to fit, or for something like the Knights pelican cases you can use a low density closed cell polyethelene foam. Check
www.hibco.com/material.htm should get you the names of some suitable products and using those names as search criteria in a search engine like google.com should give you all the info you need on what type of foam you want. Then, buy sheets big enouth to fit your case and cut them down to size. For cutting them down, I suggest picking up a hot wire knife for doing the cutting. A hot wire knife should be found at any place that sells art supplies worth it's salt. An example of them can be found at
hotwirefoamfactory.com/1_products.htmTo make it easier, you might want to buy one sheet that's the exact thickness you want to hold the parts, and then another to fill up any remaining depth, if there is any. That way, you can cut straight through the one piece and leave the other to act as a bottom, making the piece look better and more professional. If that list of previous examples doesn't help you, check with a local art supply place to see if they carry foam that you might be able to handle to get an idea as to what you want. If not, you might ask an on-line retailer if they have any chips, sample, or scrap pieces of foam they might be able to send you. You should be able to get some for just shipping charges, or at worst a small fee.
Let me know how it goes.
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