There are engineering calculations that can be done in deciding the correct profile of the seal, the size of the rib is going to have to do with multiple factors:
-Softness of the seal material (Shore #)
-Compression force (distance between your screw bosses)
-Flexibility/thickness of your substrate (at a certain point, your plastic will bend from the force of the seal and will reduce compression force)
You can look around at existing parts, but at the end of the day you will likely need more than just a copy and pasted design due to the varying number of factors. Since it’s difficult/impossible to test sealing on rapid prototyped parts, your best bet would be to try and find an outside engineering firm that could potentially help with some of those design decisions. It would save you money vs continued tooling modifications.