Airsoft Canada - Flip-up Rear Sight for A2 Triangle Sight?

So, now that I've gone against my promise to myself and bought an M4A1, it's time to figure out a sight configuration. My thought is to use a Tasco Red Dot on a riser mount but to leave the A2 (triangle) on the front as a backup. I therefore need a flip-up rear sight to go with it, but I haven't a clue what to use that will be the correct height to match it.

There's plenty of info on the web for what to use on RS, but for an airsoft backup sight on a backup weapon I'm not interested in spending 40-150 dollars for a Troy. I've looked through the AR-15 picture thread, but hardly anybody seems to post component lists, and my internet is awful.

I'm wondering about this, or perhaps even an ACM (I know) MBUS2.

If anyone can shed any light on compatibility for these or any other suggestions I'd greatly appreciate it!