I thought I would post an update to this thread, to help people who are thinking about buying malt extract in bulk.
To store my malt extract I purchased six 2.5 L, foodgrade plastic containers. This brought the cost of my malt extract up to around the same value as if I had bought it in 1.5 kg tins, but now I can buy malt extract in 25 Kg containers and get my money back that way.
Filling the containers is not as difficult as I thought. I poured the malt extract and a friend of mine held containers and used a knife to cut the DMS when the container is full. It took about an hour to fill all the containers and the job wasn't particularly messy. Each container holds about enough DMS to produce two extract brews, which is a good trade-off between storage space needed and the malt extract needed for brewing.
I then put the new DMS containers in a chest freezer to store, which also helps handling the malt extract. When I've browed earlier this week I forgot to take malt extract out of the freezer to soften it up, so I put put it in a sink of hot water which soften the edges. When came to weighing out the extra the soften edges allow me to pull the semi-frozen cora out and cut it cleanly with a pair of scissors and very little mess.
The other major advantage is the large container the malt extract came in is foodgrade and can have its cap drilled, a rubber washer fitted, and either a blowoff tubing or airlock attached, so you end up with a free fermenter.
I hope you find this information helpful, and if you have any more questions please send me a message.