Here's one from left field (or southern hemisphere at any rate :) )
I have a 40L Australian Urn and use it to make all my grain brews by using it as a hot liquor tun, a mash tun and a boiler as well. What you need is a polyester bag to fit the inside of the urn. Fill urn about 3/4 full with water and raise to a few degrees above mash temperature, fit the bag, mash in the grain to achieve mash temperature, wrap the urn in a duvet or old sleeping bag, you should only get about a degree drop per hour.
At the end of the mash, hoist the bag and drain it into the urn (a sky hook is handy for this
)
After draining and squeezing bring to the boil and use the urn as the boiler.
It's called the brew in a bag method and very popular in Aus and catching on in the USA and has made some prize winning ales and lagers.
At the risk of cross-forum fornikation here's a link to a tutorial I presented on an Australian forum. A 30 L urn is quite feasible but you might be limited to 20 to 23L brews. Also note that our big Aussie urn maker is Birko, erm a bit of a take on the original UK company one would think :hmm:
http://www.aussiehomebrewer.com/forum/index.php?showtopic=36094
Cheers
Michael
Edit: unfortunately it occurs to me that to see the pictures you would need to join the forum, not poaching members from here, but you should get the gist by reading the text, the pix were just for extra guidance.