You want fancy or cheap?
There are some nice contoured units from US Car Tool, Auto Rust Tech, and XV that like like the factory put them there. There are the MP units that bolt the floor to the connector and bolts/welds to the front and rear subs. Bottom of the line of prefabbed connectors are from any variety of Summit/Jegs suppliers that a square tub that bolts/weld to the front and rear frame sections. Home fab jobs are simply square tubing welded to the stubs.
I've also used round tubing run diagonally from corner to corner which works as a very rigid connector and the bottom half of a driveshaft loop. Causes some exhaust tube clearance issue through.