I've spent a lot of my career either heating things up or trying to cool them off. Go with a low watt density heater, not anything that glows red. Maybe a milk house heater on low, at 60C you will have to disable the internal thermostat. Temperature distribution is going to be an issue, small circulation fans will help with that, you need some air movement, you don't need a lot. separate the fan and the heating element on the milk house heater and you get both in one package. TC's are cheap if you just buy the wire and make them your self, put a few extras on the structure you are heating, you can then spot check your temperature differences there will be more than you think. Sort things out at 40C, take it up to 60C after you are comfortable with what you have, including an automatic high temperature cutoff tested at a lower temperature. Automation direct has some pretty good $100 temperature controllers. This is one of those cases when Mo' ain't better. A well sealed foam board box with a few blankets over it isn't going to take a lot.