HAPBEAR Extra Large Yoga Mat (84''x30'') - 1/4'' Thick Durable Natural Linen, Non-Slip & Tear-Resistant - Perfect for Home Yoga Workouts, Meditation, Pilates, Floor Exercises & Fitness - Free Carrying Bag Included

No help with the twin thing (you have my admiration!!) but if you need a temporary solution to make your mattress firm, you can buy a thick cork yoga mat like this and put it on your bed under the fitted sheet. HAPBEAR Large Cork Yoga Mat -... https://www.amazon.com/dp/B0F1CSH4BD?ref=ppx_pop_mob_ap_share