I have used Blue Seas Cable Clams to great success....
You simply cut or drill the approx. size and shape of the opening you need in the "rubber plug"....
Choose the size you need for application...
Clam - Blue Sea Systems
Although my solar wiring
had a round outer jacket, here is a photo
of my two Blue Seas cable clams....been working great for 6+ years and 12,000+ offshore
miles (including two Atlantic crossings, etc...)
Fair winds and good luck...
s/v Annie Laurie