Can I use a different Ground Glass and fresnel lens on my Sinar? On my Sinar the Fresnel fits on top on the Ground Glass, I hate that because you end up scratching up your fresnel. could I use a different brand GG and Fresnel on the Sinar? is the thickness of the Glass an issue? thanks. Any thoughs where I could get a 3rd party GG and Fresnel other then ebay?