If you just wanting a second battery.
Get one.
Those things are a lot more exxy than
while mostly only have a 30a or so actual battery inside them.
You paying for the electronics mainly.
Get an NZ70Z or similar.
Primarily I use my booster pack for bike, Trolley, boat trlr.
and 6x4 tyres.
Full size battery which you can keep up with solar and run van from.
My van has a "Giant" AGM 85a for $235ish from memory (on special)
Runs my part time 535 Coromal. with 200w Panel on roof. $260 del
Old reg out of shed. (I'll give him one if he wants, for the postage)
120\160w panel will do the job.
Those small power packs ok for temp work, but use them fully. regularly.
I don't think they'll last too long.
I know mine doesn't run air comp for too long when airing up Patrol tyres after sand run.
Maybeeeee. 2 1\2 tyres with Thumper..
No Matter what they say. They only a half size, at most. Battery in there.
WITH Half sized output in Usable Amp's.
As a second battery. they'll be a poor second.
Some 1in angle iron. coupla screws into timber floor of boot.
Battery.
ring of 1 in angle round top (Hacksaw\Folded and rivetted. or welded)
2 pieces of 4\5mm All thread. and coupla wing nuts.
Some wiring to suit with a Dual battery solenoid. fixed.
Anderson to car for charging.
Panel on roof for charging.
You can spread that out a bit. and have a proper instead of temp system
Three Hundred for a temporary stopgap is not an economical exercise
I reckon.
If only using for short term and has use for power pack afterwards Fine.
But not as a permanent item.