Head to Lowe's or Home Depot. Look for closed-cell foam center, shiny on both sides, radiant barrier rolls. Quite high R-ratings, but only about 1/4" thick?
Only thing is that humidity has to have some place to evaporate to! Otherwise it settles into the unpainted metal pieces welded together to make the roof sections of the body. A friend noted that when he was in Houston, in the later 1970s, that he noticed lots of darker colored cars down there, relative to his north Texas home. He was told it is to evaporate-out the humidity so it does not collect on the inner side of the outside panels, hang there, and result in "rust", from the inside out.
Whereas, in drier parts of non-coastal TX, lighter-colored roofs were the rule so as to not collect TX sun-heat, but reflect it.
You COULD use something like bats of fiberglass or rock wool wall insulation, though. Not unlike some of the panels above the headliner the OEMs used back then. More to lessen noise than insulation from heat, though.
Rather, focus on the floorpan area! Keeping engine heat out, which means "Lizzard Skin" spray-on ceramic insulation paint or DynaMat-type stick-on heat/sound insulation. Which might get a bit expensive to do a wagon all the way to the rear bumper. Not to mention the time involved!
Then using some underlayment for the wood-plank floors to replace/augment the plastic watershields behind the door trim panels. Even some of the radiant barrier stuff would be good here, provided the total build thickness was not too much.
As to HVAC air flow, plugging the gaps in the instrument panel ductwork can ensure that ALL of the cooler air makes it to the vents on the instrument panel and defroster. Whether with common duct tape or some softer foam weatherstripping, whatever works.
As a side issue, becoming more active, with doctors' guidance. Might have many benefits! When I was running regularly, even in 100*F temperatures. 5-6 mile jogs/day, doing "interval training", my heat tolerance went up big time. Made those cool showers at Bally's afterward feel amazing! Got leaner, too! But that was 40yrs ago.
Just some thoughts,
CBODY67