In order to get a good internal figure, you need to cumulatively deposit about 1-3 microcoulombs of charge per square cm inside the acrylic. Depending on the beam current, it may be necessary to chill the acrylic to reduce charge leakage. we have used dry ics chilling for making Lichtenberg figures on lower-current research machines.
A couple of papers provide some practical information on using medical accelerators:
"TECHNICAL NOTE: Production of electron trees with a clinical electron accelerator" by D. Pomaré, K. Stansfield, C. Lewis, and M. A. Ebert, Australasian Physical & Engineering Sciences in Medicine Volume 27 Number 1, 2004
"Lichtenberg Figures (Making them using a Varian Model 2100 Clinac)" by Markus Arn and Wolf Seelentag, Swiss Society for Radiation Biology and Medical Physics. http://www.ssrpm.ch/old/lichtenberg.htm
Good luck - let me know how it works for you!