Resin casting questions - The Second Coming

Hey all; after a week of panicked mass creation just before Anthrocon back in July, I managed to get my sculpt not only completed but a mold made as well. Night before AC 2015 I was frantically mixed and poured two sample pot sets worth of resin into my mold and made a pretty decent cast. However, at around 4 in the morning I realized that while following qarrezel's tutorial I didn't know how many layers of resin I was supposed to do. I didn't use URE-Fill III in my mix because I didn't have it. I was told by a friend that without the filler just 3-4 layers wasn't going to be enough structure to make sure the mask was properly sturdy. He told me I should use around 10-12 layers without filler. I've decided that my initial cast is definitely too fragile around the edges, as the back edge is close to the thickness of a couple of sheets of paper.

So my question is. How much resin is generally used in a casting, and which type do you find stronger/more preferrable? I'm debating between Smooth-On's Smooth-Cast 300 and Smooth-Cast ONYX. If I go with the Smooth-Cast ONYX, is it necessary to also use filler since it's got a harder wall thickness? I'm guessing the inside of the mask should be black so light coming in through the eyes doesn't reflect off one's face and make them visible. However black holds in a lot of heat and might make furring look weird. Any advice?

Here's the resulting resin base.