The only things I had to buy was the mesh from http://www.inoxia.co.uk, I bought two 30cmx30cm pieces of the #30 mesh, about a tenner.
Also two M14 stainless washers from http://www.a2stainless.co.uk, these fit snugly round a 15mm pipe.
You'll also need a very small length of 15mm pipe, a length of 15mm pipe long enough to go from the middle of your boiler to your tap, a 90 bend, a T piece, and a nut and olive of suitable size to attach onto the back of your boiler tap.
Use a plate or fermenter to draw a circle on one sheet, on the other sheet use something to draw a slightly smaller circle, about 1.5 cm smaller all the way round.
Cut them both out, a decent pair of scissors do the job, watch the ends of the mesh, it's very sharp and it draws blood really easy.
Fold the larger of the pieces of mesh in half then half again, snip a very small quarter circle off the end, when unfolded it will give a hole for the 15mm pipe to go through and it'll be in the middle of the mesh. If it's too small fold it back up and snip a tiny bit more off.
Solder the small piece of pipe into the 90 bend the put a washer on, use flux to help the solder flow.
Push the pipe through the hole in the larger mesh.
Turn the mesh over, put the other washer on the small piece of pipe, then the T piece, push it as tight as it'll go together then solder this.
Now put the smaller piece of mesh over the T piece. Start folding the larger piece over the smaller bit.
When you've gone all the way round crimp it flat with a pair of pliers.
Then fold it again over itself, again crimping it tight all the way round.
It may not look totally circular, it's quite difficult to get it perfect but it won't make any difference to the performance. Some folk have sewn theirs with strands of mesh taken from the edge, I tried this, had blood all over and found it unnecessary. Mine is made exactly the same way and hasn't budged in over 30 AGs.
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