Cropzy here again.
Here is how I made my gun case.
I purchased one of these of off Fleabay http://www.ebay.co.uk/itm/271152550770?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
Now they are old and have been stored outside since the 90's so they have a little rust on them.
The case itself is quite heavy being an army surplus ammo crate. They are air tight and water tight and the locks are very tough and hard to lock.
Mine came with the original inserts for the munitions it once held, which are certainly bigger than 50 cal.
I took the lid and give it a scrub down with a wire brush to get rid of all of the loose rust and then did the same to the case itself.
I then took the lid and painted it with some green Hammerite we had laying around, this is so that the lid is protected from rust and rain etc for the future.
After it had dried.
I then started looking at foam to put in it, I figured most gun cases have egg box style foam in so I bought a sheet of it measuring 2m L x 2m W x 3mm H and cut it to size in 3 pieces. 1 for the bottom and lengthways sides. Then the other 2 for the other sides.
I then purchased some spray glue (carpet adhesive) and sprayed the inside and fitted the foam and then let it dry.
The final product turned out quite well, its not as good as a professional but it will do!