[asid61]

It's hard to get a swerve as light as a mecanum. Getting one to that points could take iteration and a CNC to make optimal lightening patterns in aluminum.
For ideas, look at 2451's cim-in-wheel swerve drive, or their two speed swerve drive design posted last year. Aren Hill also developed a tiny swerve module and a single-speed design similar to 2451's, as well as countless other designs.
If you don't have a CNC, I would recommend buying the Revolution swerve modules from team221.com and fitting them to your application. Those can weigh very little if you avoid a lot of external gearboxes and such, and only require a little work to integrate into a chassis.
Alternatively, for a fast and easy solution, the AndyMark Swerve and Steer is a fantastic choice as it offers all the motors, gearboxes, encoders, and the module itself for less than $350. And it comes already assembled. For a first time swerve, those are the easiest, however, it is not the lightest solution out there.
My team might pursue a custom swerve design in the offseason, but it remains to be seen. We tried it last year but bit off more than we could chew in the machining sector and could not complete it by build season.