Many passive DI boxs are just small 1:1 transformers with appropriate connectors...

But your RFI problem puts lots of questions in my head, however I'll limit to just a few.. Are we talking a real transmitter (say for FM narrowcasting) or a digital thing like Bluetooth or Wifi ? Are you using the correct antenna for the transmitter? For earthing purposes how are you powering things, all off the one power board or via various GPO's around the building? Have you rack mounted all that can be rack mounted?

Usually the suffering of RFI can be due mostly attributed to a bad earth plan or an antenna that has poor VSWR. Most new gear these days has quite good RFI protection and will only present symptoms when other gear is having problems. As an example, at one time I had a 20W VHF transmitter in a rack on test in my workshop and a Behringer UMC 204HD USB audio interface operating fine without any RFI into my Reaper loaded PC all located less than 2m away, though I did have to pay special attention to grounding and avoidance of ground loops. So it can be done, though even simple things like that 'old faithful' or that one 'cheap' power supply can bring a whole lot of unique grief to your party..