I have done the same with an spare refrigerator in our garage. I put together an ardunio controller with 100W bulb. I can liquify 5 gallon pails in 2 days at 100F.
What size bulb are you using and are you sure it is getting up to temp? 100W light bulbs are getting hard to find around here. Most of the time the advertised wattage is some type of "equivalent to" number and by reading the fine print you will see the actual wattage is much less.