At your local pharmacy or grocery store buy ThermaCare menstrual heat wraps (it sounds strange, but its low heat feels so good, and the regular ones might get too hot). Next, about two hours before you want to masturbate, attach the wrap to some snug-fitting underwear (one size too small). Make sure it goes from your penis to under your balls. It takes 30 minutes to warm up; it doesn't get too hot, but it feels really good. You will be thinking of sex the entire time your wear it. Wear some baggy clothes, because you may have an erection most the time.
One item I have not seen mentioned to simulate a vagina is a simple and wonderful ready-made product: a Hot/Cold pack. It's a plastic bag about 10 inches long and 4 inches wide by maybe 1/4" thick. It is usually filled with a blue gel. If you let it soak in hot water for a few minutes, it gets to body temperature. Then you can squeeze some lube all over your penis and wrap the "pack" around your erection. You can make a sleeve out of a sock or just use your hand to adjust tension. This pack makes a nice and easy humping or masturbation experience, and it's washable and reusable.