On my Echo1 G36c I installed a BSA Red Dot Sight.
It Was $30 at Wal-Mart. It has a 30mm opening on BOTH sides, can be outfitted with flip covers, and is designed for REAL guns, so it's considerably durable. Adjustment is fairly easy, I had it dialed in in a few minutes. Plus it has 11 intensity levels, so you can find the right dot for your light conditions. It uses button cell batteries.
It doesn't look quite as good as a reflex sight, but I like it personally.
If you want to see pics, PM finn1016. He can post some for you. And if you have some other questions, feel free to PM me.