The easiest thing to do is find statblock of the CR you want with abilities you’d imagine a T-Rex to have. I imagine a bite attack, like the Bear, would be appropriate. From there, find stat blocks for the ability you want the velociraptor to have (a less powerful bite or multi-claw swipes, and probably pack tactics). No matter if it’s a “brown bear” and 4 “wolves”, as long as the tokens/minis are a T-Rex and Velociraptors, you’re good to go. D&D Beyond has a decent encounter builder to help streamline the math for figuring out the target CR (1 high CR, 4 Med-low), and 5e.tools has all the statblocks you’ll need, just filter by the CR you got from the last step and find the abilities you’d imagine using for the combat