What are you using to check the temperature? All those heat pads should be plenty enough, try setting the bin above the ground floor in the garage. Like on a table or something with a blanket underneath so the heat doesn't escape underneath. If you are using like a gauge that's planted on the side of a bin, that's not going to work.. You need to measure the flat bottom where most of the heat is to get an accurate temperature, the heat dissipates fast. How big is the cut out on the top of your bin? Does it take up the whole top? Yeah you can use a heat lamp hovering over, but that shouldn't be necessary. I can only keep mine in the garage and I live about an hour from you. They're on a card table off the ground with a 30-40 gal zoo-med uth on the bottom and that's it with a zoo-med rheostat set almost to max. Temperature should be way higher for you than me... doesn't make sense.

Update me with some pics of your setup, by the way your neck/back heater does it have a thermostat that shuts it off after a certain temp? Does your wall outlet not support the amount of wattage you're using an have a socket limit?