How to respectfully bow out of interview process?

Interviewing for a firm where I was given a strict deadline for a modeling case study (hour long presentation + 30 minutes of questions, 4th round with one to go) of 36 hours from receipt. I have started it but simply think there is slim to no shot of me being able to complete this case study to their specifications in the next day and am not fully vested in this opportunity as is. Given I am using a HH who has sourced other opportunities, I want to bow out as gracefully as possible to make sure the firm does not speak ill of me. What would be the best way to end the process? should I say I am unable to complete in time and want to be transparent with them? For context, I have completed a similar case study for a different firm, but they gave me five days to do it. 

Thanks for any advice!