I have a regular marine with me at the moment, and some K&J DISC D201 Grade 42 , 1/8" x 1/32" magnets. These work well But were not as strong as I would have liked to use on dreadnought arms. (Just to give an idea on strength) They held them on well, but the arms would move up and down to easy.
The D201-N52 of the same size is stronger. Not sure how heavy a jump pack is. I would think either of these would be what you want.
They are just a tad bit bigger then the hole in the backpack. So easy to install there. However in the back of the marine you don't have much space to make a mistake. You'll have to be pretty spot on. If your off just a bit the packs will hid it so not to big a deal.
You could probably leave the nub in the marines back, and put a magnet just above it. This would help keep the packs straight. Also would add a bit more support. Looking at the model I think it would be easier to install them. Since there is that depression above the nub. The backpacks have more space to drill holes.
Magnet link
http://www.kjmagnetics.com/products.asp?cat=10