lightweight
I like Power Master. You can get them with bolt on mounting brackets. I prefer a 100 amp Denso style. Not much heavier or bulkier than the 50 amp ones and a lot more capacity for those later rounds when external charging is minimized.
The 50 amp ones don't do much at idle either. Check out the excitement RPM. This is the minimum it will start charging at.
I run 2 batteries and the 100 amp alternator and never need to charge in 7 rounds plus practice runs. I do trickle charge during the week just for piece of mind.