I have set spring loaded ball bearings through the floor plate to "click" past so those old fashioned levers will stay shut. You make a small cylinder with a ball and spring that can go through the plate with only the ball sticking up. The lever "clicks" either into or over the ball and stays put. Easy to make and they work well The cylinder protrudes into the b\mag box a bit so it must be short enough to clear the magazine spring, but you'll find that you have about .220" to .250" to work with and it's just fine About 1/8 is used inside the plate itself and what ever sticks up clears the spring because of the shape of the mag spring itself and the small recess within the plate on the inside. I know of a few done that way that did cause the magazine to loose one round of capacity, but the owners thought 4 round that were 100% reliable were better then 5 rounds that sometimes go "bomb-away".
Depending on your caliber, some Mausers have a bit of "give" with 5 rounds in the mag and some caliber fill the mag 100%. Those that give a bit of space can be made to work with no problems, but those that fill the mag up close to being "tight" with 5 rounds will probably cost you one round of capacity to make this modification.
