Does anyone know of a good method of dropping cable sizes down in limited space inside a panel.

This is related to my earlier post, where for example I need to upsize cables in trunking to account for grouping factors, but the larger cables on reaching the panel need to be reduced so they fit into the terminals on the breakers and/or the outlets as relevant.  

A common method as I understand it is to have a termination chamber with terminals/short busbar for each connection but we have limited space in the already manufactured panels to do this.  I have seen some DIN-rail terminals with dual studs on that might work if we can get some rail fitted into the box.

My ideal would be an inline butt crimp that has say 240mm2 on one side and 95mm2 on the other (or other various smaller combinations).  Does anybody know of any such things - or anything else nifty for this sort of thing?

Thanks.

Jason.