For a Mk D, the two most popular places are:
(1) in the corner, at the back of the offside wing, on a "shelf" supported by a brace between bulkhead and botttom of wing, and
(2) in the "boot'
Version (2) allows a bigger, heavier battery, but needs decently thick wire so's to avoid voltage loss.
My G (much the same starting requirement as a Mk D if uprated to 250cc) uses version (2), with a 42Ah
Extra Ah comes in useful if (when?) you're out somewhere and the Dynastart decides not to charge ...