Would adding ~2 ft of copper wire to extend high voltage power supply cause problems?
Since the peak voltage of 220-230VAC mains is about 320V, and you use 300V maximum, any standard mains rated cable should work just fine. Any other wire rated for this voltage should also work (like those banana plug multimeter wires which are strewn all over the desk in most labs).

However, the manual for your supply says that it will go to 8000V. Thus the above cable shopping advice only stands if you never set it above 300V... that includes software bugs, interns, cat sleeping on the keyboard, etc...

Likewise for the connections...

The manual also says "The cables in our standard cable sets each include a 10k-Ohm resistor to protect the HVS circuits from accidental overloads and shorts."

Make sure you're aware of the resistor, voltage drop etc...

Now, your current setup is a nice way of killing someone with bare and live wires next to water... especially with a power supply capable of 8kV. You should review safety, make sure all metal that can come in contact with the solution is grounded, and keep all metal spoons and stuff like that far away from the pot! 

Since the connector on the power supply side seems custom, you probably won't be able to use any cable you want. Heatshrink on your nice expensive high voltage clips would be a problem. So, purchase one or two transparent acrylic box(es), drill a hole on each end, run the wires through the holes, and make sure all the bits of exposed copper are safely inside the plastic box... It's easy to recover your HV clips for another experiment, and should be safe.

A transparent box makes it easy to notice if something comes off.