What's the acceptable method of splicing 3awg aluminum wire inside a junction box?

I've got a 408V 3pole disconnect that's overheated from oxidized contacts. Wire insulation is burned off inside the switch and the plastic plate looks like a dali painting.

I would like to add a junction box at an existing conduit connector a few feet above this box and only pull new cable back to this box instead of pulling new cable all the way back to the mechanical room about a hundred feet away.

A split bolt connector and friction tape was suggested, This seems like sort of a home brewed remedy, are there any connectors made for this?

Thanks