One solution is using a black pop-up portable pod like this one one from Amazon. Blackout tents marketed for people with special needs cost hundreds of dollars, but this one is marketed as a dressing room and cost me $39.99. (It needs a wall nearby for stability, since it's made for staking into the ground...wouldn't work for horseplay inside.) With pillows around the sides, a long cushion from an outdoor lounge chair to sit/lean on, and a body pillow across the doorway, the feeling is very similar to the inflatable ball pits. Soft is important so Mason feels safe. I think he approves :)
And the black walls make for an extra layer of fun with toys that light up!