Control pinions, by their name, control fuel output of each pumping element.
The pump side cover has very low pressure, only internal engine blow-by pressure. It prevents splash oil and some gasses from leaking.
It would be difficult to make a rubber gasket. I suggest using the old one and putting good oil resistent sealant as a temporary measure. You will have to clean off the sealant when you get a new gasket. Bosch does nor recommend use of sealant in this cover. Also, be careful in using too much sealant on the side cover, because it could flow in, strip, and interfere with the control pinions.
Best thing to do is look for any ADS (Association of Diesel Specialists) service shop in your area, and get the gasket. They are fuel injection people and are all over the US, they have a website too with membership directory and location.