Convolution matrix for converting 1080i to DVD?
Really nice question, common problem.
After Effects, Autodesk Combustion and Autodesk Smoke can bring you a software downconversion(1920x1080 -> 720x480) with more quality than Final Cut Pro or Premiere Pro. Smoke can make it near perfectly. 

You can use (buy/rent) hardware downconverters, like AJA Kona 3, or Blackmagic clones. Better results goes with AJA. 

There are even more expensive/professionals solutions like Sonic and others which will make the MPEG-2 compression directly in hardware too.

If  you don't get your matrix filter from someone (not me, I'm not skilled for this kind of mission), best approach will be small tests with software and settings, tests, tests, tests, and then setup the workflow to not change forever!

Good Luck!