Need to use the correct part number or designation for this thread to be useful.You can get in a muddle without specific part numbers.Here is an example:
Which OEM size battery options are there? - 6SpeedOnline - Porsche Forum and Luxury Car Resource
It used to be simple - just use the familiar BCI chart to find dimensions/polarity and determine interchange. For example look at 94R here:
https://www.batteryweb.com/bci.cfm
Then came DIN and EN to complicate things with multiple numbers and letters for the same item !H7 and H6 in our case.
European Norm (EN) Battery Design - WORLD SOURCEONEŽ
And no, DIN doesn't always have an EN equivalent.
So you have to drill down into the BCI dimensions/specs to find the sizes that fit:
http://www.batterysales.com/downloads/battery-replacement-data-book-1994-2013.pdf
The short version is that the Boxsters from 1997 to 2005 take Type 48 BCI
Later Boxsters take 94R - see page 170 here :
https://parts.cat.com/wcs-static/pdfs/PEGP7801-06.pdf
If you want to fit a lighter+smaller Odyssey battery,they don't have dimensional equivalent to the BCI48 nor (4R,so you need the BCI dimensions & then look here:
Odyssey Battery - Wholesale Odyssey Batteries
PC1220 may be a good substitute?
http://www.odysseybatteries.com/batteries/pc1220.htm