I'd avoid using a clamp lamp, then you will be dealing with humidity issues.

another option for you is ordering an ultratherm heat mat from beanfarm.com its basically the same material as flex but it doesn't stick to the bottom like some of the other heat mats. makes cleaning in the tub a snap! and it has a thicker layer of plastic laminate for extra safety/durability. the site claims you dont need a t-stat with them but i wouldn't chance it w/o one.

when i first started out, i used these mats with an Alife t-stat also sold from beanfarm. they aren't the greatest but work until you can get a better one. or you could get a light switch dimmer. I've never went that rout but from what I hear they are about 10 dollars from your local hardware store and people have had good results with them.