All you need are your linemen and some 25lb or 35lb weights.
Get players into three or four equal lines. Have the first group step out and get into good pass blocking position with their butts down.
On the command have the players start chopping their feet and punching out with the weights in their hands. You can have them stay looking in one direction or have them turn on command while still chopping their feet. This drill is good for building arm strength and making them bend their legs. You can time them or just have them do it for as long or as short as you want.
Make sure they keep their butts down and their arms fully extend. If the players start forming bad habits stop the drill or make them do it right.
This drill is great for punch strength and leg bend. This drill will make them work really hard and is great for the off-season.