Seiza bench for yoga?

[riplo]

So I've been meditating for a good while now, and my preferred way of sitting when I do so is on one of the benches (or a similar structure I arrange out of whatever objects are around)  shown below, sitting with my legs tucked under. I'm very inflexible and this way of sitting is perfect, as over time I've already flexible-d those specific muscles or whatever required to sit like this. It's pretty hard to fall asleep like that, as opposed to sitting normally on a chair, and it naturally keeps your back straight. 

I have tried to begin a Kriya yoga practise, sitting in the same way as Leo describes in his  'How to sit for Kriya' video on the blog. I've been doing the practise for only about a week now, and I'm able to sit like he shows reasonably comfortably, but after literally like 5 minutes my legs go completely numb from cut off circulation, and seeing as I try to sit for 25 minutes, it doesn't seem like a very healthy way to sit for the practise in this case.

I know that over time my flexibility will increase, but will circulation in my legs (ie will they stop going numb)? Is it a better idea to just use the bench, or is there an actual reason for why I shouldn't do so for yoga specifically?

Thanks guys x

[voxun]

Doesn't matter what matters most is to sit up with your back straight.

There is a kriya yoga teacher and he just sits on a chair. Otherwise you need to spend months or longer before you can sit comfortably.