Flexwatt can be used, but I would insulate it and use a thermostat. Here's a link to what could happen if a thermostat isn't used:
http://www.arbreptiles.com/cages/flexburn.html. I'm not sure how thick your acrylic cages are, but ours are a minimum of 1/4" and at between 350-375 they become pliable, and can melt at 400. Lamps and bulbs can easily reach these temps, so don't put them directly on or against the acrylic. You can cut a whole in the acrylic, cover it with metal screen and put lamps on that if you wish to use them.
Important note about cleaning it: Don't use window cleaner or anything with ammonia! Use a mild detergent, hot water and a soft terry cloth. Avoid abrasives, as they may scratch.
I'd love to see if there's anyone who has other heating options.
Hope I've helped.