What and why do you think you need to test a 61E? What are the goals of the testing ? What are expecting ?
FWIW: I never heard of anybody bothering to benchtest any SOHO FW.model with a fortitest or anything else. I would investigate if NSSLab has any results or testing guidelines and start from that point
I think the OP wants to make sure all the electrons flow in their right lanes. Seriously, a burn-in test for a desktop model is IMHO a bit out of proportion.
One could do it oneself in a lab, the only thing needed is one or more traffic generators, some config with policies, UTM configured, a reporting instance (FAZ)...and if all you need to see after 3 days is that the FGT still is processing data you wouldn't even need a FAZ. Cost of equipment ~50.000 $, cost of FGT ~ 500 $.
To avoid a DOA delivery it would suffice to power the FGT up, configure it for a couple of hours (slowly) and leave it on. If any crucial component, like the flash memory, the CPU or the plugable power supply had a flaw it would not survive this time span.
Maybe your customer would be more assured if the FGT was equipped with a service plan with defined SLAs. Any hardware will break eventually, and it would be good to have a plan if it does.