Jimp, the ammo boxes is a great idea. Easily moved as required. It also passes the everyday test. Easily placed once things start looking bad. Bad thing is they tend to not stack like bricks very well..but I might have a solution.
I'd agree with Mr Dare though, use sand. They would be like metal sand bags.
If I might suggest, place a piece of foam or something where the holes from bullets passing through "close", on one side of the inside of the can, the "outside". Then paint the "outside" a distinct colour (this side towards enemy). This should help keep some of the sand in the box once holes start to appear in them.
I'd think that some sort of retention method would be in order. If the boxes start falling down, especially while you are taking cover behind them, they can really hurt. If they were "permanently" located, then some sort of revetment would be in order. Use something like corrugated metal around the inside of the "foxhole", forcing the cans against the wall, and this in turn held in place by steel pickets, say wired and then windlassed to the wall studs. (windlassing, for those new to this term, is when you take a piece of wire wrap it several times around a couple of strong things a distance of a couple of feet apart, insert something strong into the opening between the strands and twist them together, causing them to wrap around each other and become tighter. Used in fencing of all manner)
Bug netting and 2 x 2's can be your friend too. Build a structure inside of your window, sort of like a closet but opening towards the window, and attached the bug net (door screen, but not the metal type) to the inside three sides, like a curtain. This will act like sheers. You can see out, but unless there is a light on in the room you are in, they can not see in. Beware of other windows in the room. You can shoot from behind this (like, a few feet behind it) and the people outside won't easily detect where your shot came from. After shooting a few rounds, move to a new location. Eventually, any dunder head will figure that just blasting away at a window will probably hit you. Stay low. Pre-position ammo at each location. Yell to others in the building. Alone? Fake it. Just yell from different locations to your "allies".
Both suggestions come from an older "Urban Warfare" manual. Maybe US, Maybe Canadian.