Just get some perspex in the right size? It's fairly cheap to buy and there's bound to be a web-site you could order it from pre-cut to your required size.
After that it's an issue of what exactly you need, and how handy you are; for example you could do a full replacement for the metal side panel, but you'd probably need to cut out some tabs so it sits properly in place and either use a rubber strip for a friction fit, or try to make something compatible with the Mac Pro's built in latching mechanism (it's not a hard job, just very fiddly if you've never done anything like it before).
If you're happy only covering part of the machine like the G5 shields did then you can easily go for a smaller piece and maybe use some kind of right angle brackets to hold it in place, or even add something that can slip just under the hard drive trays.
In all cases, rubber strips for noise cancelling are a good idea as it may be hard to get a perfect rattle-free fit with a DIY shield.