Just buy the thing.
I tried years ago to improvise. It ain't worth it, and your really taking more risk in damaging the case.
Splitting cases is risky somewhat anyway. Getting them back together and spinning smooth is the other 1/2 of the battle!
L@@K at the pretty pictures of them, you could probably build the thing. it isn't that complicated. but why?? case splitter
I used a piece of 1/4 inch steel and bent an offset. Welded a nut on it, and drilled a couple of holes for the bar studs. But, it would bend. I welded one together from parts on eBay that works like a C clamp.
Works OK.
My project 372XP came apart without a lot of drama, though it wasn't assembled with any sealant on gasket.
I used this crank knocker that screws onto the flywheel threads (Husqvarna) and is separated with a few mallet hits. I don't do this for a living so didn't want another single-use tool taking up space.