Couple ways to handle it. You could cut holes to line up with the existing holes. Then, you can use 1 bulkhead for each hole to connect the sump and refugium by adding a gasket between the tanks. You could then pump water up-and over and let it flow back into the filter, or maybe even push water through one bulkhead with a pump and let it flow back in via the other two openings (not my favorite idea).
If you want a higher water level in the refugium, then cut holes near the top of the refugium tank and use them as drains, and have a pump in the sump to pump water up into the refugium. This is how I would do it.
Either way, whatever you rig up, put something like egg crate in front of the drains so a clump of algae doesn’t come free and clog a drain.
