I have a very small above ground cinder block house built in the 40s. My bathroom was built in what used to be the backporch so it’s about 5ft widex8ft longx6ft8inch ceiling. I have a 60x32” full bathtub in the outer corner. There is an appropriately 20” x20” access panel on the nonplumbed side of the bathtub. Bathtub is acrylic over fiberglass. Problem is to fit the tub we had to cut into the studs behind it and it’s all bare cinder block wall and concrete floor, so it’s a very tight fit and I’ve noticed that the bathroom and specifically the tub area radiates the outside weather. I live in a hot and cold enough climate that it’s miserable about half the year.

I have shoved some insulation boards into the gap in the front of the tub and behind the access panel but i cannot fit the boards into the gap behind the tub(against the wall)

Are there any reliable options to insulate the wall without a total tear out like spray foam or flexible insulating sheets perhaps?