Empire Canon or Mortar
Have you ever noticed how games-workshop gives lots of stuff with all their plastic kits. In most cases this is nice, but not very useful except for conversions. But if you look at the canon/mortar package, they give enough stuff to actually construct a canon and a mortar (after a bit of modeling work).
If you look at the parts needed to construct both the mortar and the canon, you'll find the only part that is used for both is the base of the barrel. This base is actually a very simple construction, which almost anyone can make in under an hour.
Requirements:
a small piece of 2mm thick material (wood, plasticard or even cardboard)
a small lath of wood which is about 4mm wide and 8mm high, you'll need somewhere around 5 cm of it.
some thin cardboard
(optional) Some Green stuff
Instructions
First of all, take the 2mm thick card and cut a 19 x 19mm square out of it. Make sure it's as straight as you can, because this will be the base of your canon, and if it's crooked, the whole thing will look it.
Next we take the wooden lath and cut two lengths of 24 mm from it. Then glue it all together in a |_| form. (See the official base). Allow this to thoroughly dry.
The sideboards will have to be prepared to hold the barrel. To this end take a drill (preferably a dremel or likewise tool). Use the drill-bit sideways to cut half a circle into the sideboard. It should be about 16mm from the rear end of the construction. If your drill bit isn't large enough, just wiggle it about to make sure the barrel will fit.
Then take the (thin) cardboard and cut some 2mm strips from it. These are glued around the sideboard and should look like the iron strips found on the original.
After this you take the barrel and glue it into place (and the barrel-stand in case of the mortar). Once the barrel is in place you again take the thin cardboard. This time you cut strip of 4mm from it. This strip is placed over the hinging of the barrel (See the original one for more info). This can also be done with green stuff.
Your result should look something like this:

After this it's ready to be painted and fielded in battle.