You could have it attached directly to the thigh armor. Of course on the fromt, but have it over lap a bit then use a strap to help secure the entire shin armor to the thigh (one of two or more of course). The holster can he made out of syntra or whatever material you like, and make it so that it follows the profile of the blaster.
You could use something like a covertec clip, but one with a quick release so that you don't have to fumble around with pressed a button or release it, just slide it out.
You could have the grip end angled out a bit for easier deployment. With a straight up and down holster you have to pull up to get it out of the hilt and then point/aim/shoot. This way, instead of pulling it up and out, your just basicly bringing your arm up from your side in one sweeping motion. Either that or some quick release that gives the same one motion sweeping action. It may only give a person 1-2 extra seconds, but that counts in a shoot out.