You can get one to fit the heater motor hole, you can also get universal ones and make your own hole. Or you can make a hole and put the lines through with grommets, that eleminates two places for each line to leak.
Be sure you don't fill holes you will need later.
How are they being filled?

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-