Depending on the model there may alreay be provision for a boot light in the "top of the range" model. In this case the wiring will already be present and probably on a switch and possible recoding of the chassis control unit wold be required.

Are there any cut outs in the boot that could take a plastic lens? If so then much of your work could already be done.

Q.

If boot lights were fitted to some VW Polos your best bet might be to try the local scrapyard. You should be able to source all the bits you need and see where and how they fit. Don't limit your search to Polos as other VAG models might use the same light.