Go to www.brownells.com
and search under part number 100-000-618.
This is the Weldsmith bottom insert for the Rem 700 LA. THis is the bests insert I have every used. It is a very quality fit to the mag port and is perfectly profiled to match the outside contour of the Rem 700 receiver.
Take your receiver, degrease it completely in the mag well area. Then take a high quality epoxy, I generally use a high strength Clear 5 minute epoxy for this. After degreasing the receiver, mag well do the same for the insert. Be sure not to get it oily once you have cleaned it.
Mix up some epoxy, and apply it to the receiver around the mag well machining. No need to over do it, just make sure that along both sides there is a coating of epoxy.
THen carefully drop the insert into the mag well and hold it down against the receiver so it sits flush. With some Q-tips clean out any excess epoxy that oozes around the insert.
Then quickly and while still keeping pressure on the insert holding it solidly into the receiver, turn the receiver over and clean out the excess epoxy inside the receiver as well as this will prevent the bolt from passing if not cleaned up.
This is why use only what you need to get the job done or the clean up will be dramatically more work then it needs to be.
Turn back over and either apply a few wraps of wide masking take or put some weight on the insert to keep it solidly against the receiver. Will set up in 10-15 minutes, ready to shoot the next day.
The reason I use the 5 minute epoxy is so that it sets up and does not run all over hell in the receiver. The longer set up time epoxies are generally stronger but I have NEVER had a problem using the 5 minute epoxy as this is a very non stressed area. Just be sure to degrease everything well before starting. Like everything else, cleanliness is next to godliness!!
Here is a pic of what the finished result looks like. This can DRAMATICALLY increase your bedding surface if you fill in the mag box on your stock as well. Also greatly stiffens up your stock so you should get better consistancy all around.