Bojan, that is going to be one very solid dome! Your choice of materials should provide a good measure of thermal insulation too. My 3V dome from plywood (without any frame) is very rigid - even after the aperture opening was cut. Some pics here
http://www.iceinspace.com.au/forum/s...t=36222&page=2 (see posts ~#44)
Have you worked out the width of your aperture yet? Don't underestimate how far off-centre a GEM will put the scope! I'm almost on the limit for mine (at nearly 700 mm), but a narrower opening = more shelter for the scope and operator.
Looking forward to more progress reports.
Chris