How can I reproduce ambient EM fields inside a Faraday enclosure?
Accurately reproducing ambient EM fields inside a Faraday tent is going to be very difficult. I know you say you don't need perfection, but using a couple of antennas is probably going to be way off.

An alternative and likely much easier method would be to have two tents, one a Faraday tent and the other similar looking but not - you could cover both in lightweight fabric to hide the distinctive foil/mesh. Move the subject between tests.  Or swap the foil door for a fabric one, and use a divider inside the tent so the subject can't see which is which.

If you are trying to reproduce the fields electronically, you'll want several broad-band antennas to cover the full range and you'll want to measure inside and out with a spectrum analyser to verify you've got it right.  Some kind of RF graphic equaliser made from amplifiers and notch filters might help tune the system and get them the same.