How can I dynamically introduce different delays in a physical ethernet link?

Hello r/networking!

I am currently working on writing some controller applications for SDNs that take into account multiple parameters for moving packets around the networks. As a requirement for the project I have to use actual physical links and not just artificial links such as those generated by mininet (which allows controlling the link parameters).

My big question is how could I add some delay on a physical link (preferably not a very expensive solution but at this point I am desperate for an answer so any idea will do). I will need to be able to add the delay and remove it dynamically (in an ideal scenario) and see how the controller reacts to these changes in the physical layer, that is my end goal.

My only current idea is to use an FPGA and just buffer the signal for a few milliseconds, sadly this is far beyond my FPGA programming skills and I am looking into alternatives.