I've measured up and I'd need 8 lots of a 40mm x 54mm mesh. The holes would need to be around 2mm square, big enough to let water through, but small enough to hold the cutlery.
We'd looked at a replacement a few weeks ago, found an original on Amazon and it was £25 - £30, which made me rather vexed. I wanted a cheaper alternative, but the cheaper non-original ones didn't have the deep grooves which allow it to fit over the racks. The product link's here.
I now see that the current prices are somewhat more appealing. :-$
If Ant or Throb are willing to make a mesh insert with postage for less than £9.20, then I'd much rather send the cash their way than Amazons (or a third party). :)