Midway had 7 on clearance for $156 or so when I posted this link, all gone now.
The magazine hangs pretty low but I don't think you would have a problem prone unless you like to use a long bag like a Bud's. There may be a 5-round magazine available but I haven't seen it yet.
There is a bit of inletting and fitting required, no idea if they will work with other than Remmy LA style actions.
Glen Seekins makes a hell-for-stout detachable magazine and JW Precision is now selling the V-Bull mag system as well.