Mancini rear seal.

I used that one once and it works fine. But, I'm of the camp that believes there's nothing wrong with the factory ones either....seal issues are 99% of the time due to the install regardless of which retainer is used.
I use the red Viton/flouroelastomer seal and NO side seals at all. I dry-fit everything first just to make sure the bolt holes will be aligned enough with the retainer down. I let the seal dictate how the retainer sits--it should sit centered on the crankshaft. It may not if you just line the bolt holes up and tighten away, and there can be a bigger gap on one side of the seal. That's gonna be a leaker. Sometimes (very rarely) I've had to slightly enlarge one or both holes so the bolts fit nice with the seal properly located.
Clean all mating surfaces obsessively and don't get anything on them during final installation, no oil, sweat, or slobber....:D
When it's go-time I fill the side grooves with ultra-gray, and do light smear on the side and bottom mating surfaces. I let it set up for a couple minutes and gently slide it down while letting the seal dictate the final position...be gentle and let it kinda"wiggle" itself home. Don't worry about the silicone that oozes out, you'll clean it off later.
I then put the bolts in and just evenly lightly snug them.
Walk away..
Final torque them the next morning, done. I'm sure everyone has their way of doing them, but I've never had a leaker with this procedure, with any retainer.

I also TEST the seal first if the engine is out..but it sounds like you're doing it from underneath. Good Luck!