Like the idea! I think carriage bolts and wing nuts are the go-to hardware candidates but I think some kind of additional blocking or stop cleats would be good insurance just in case there was some movement if the carriage bolts and wing nuts loosened up. Another thought is to incorporate T track (as used for router tables and other woodworking jigs) as a more durable alternative to a through slot in wood. Jig knobs could secure the pads, or even cam clamps with some kind of additional strap to ensure the cam end doesn't pop up. Also I'd be curious to see how the object would be secured into this frame - would there be a matching lid with adjustable pads to match the ones you show? If the sides were deep they could accommodate adjustable pads to hold down the object.

BTW I've been using Sketchup a lot lately with a modular wall building project, it's great to use to think through how to assemble things such as this.