I only use aluminum sheet for the floor to allow the keg box to slide easier. It has virtually zero R value.
Freezer plates vary quite a bit, some are open in the back to allow air flow through them, some are closed. In the case of a closed plate, blowing air IN is not very effective. In those cases, use TWO fans mounted next to each other, one blowing IN and one blowing OUT to create a circular air flow through the freezer area. If you have frost, that means the plate is 32 or below and that’s a good sign.
In my case, I have what I call an open freezer plate so I use just one fan, however after a few months (especially in summer when humidity is high) the plate builds up enough frost to completely block airflow and I get less air flow. When this happens my temp climbs to around 50, just like you are experiencing.
I have found that if you use a single fan, it’s best to blow OUT, not in.
One other consideration…how hot is the back of your fridge?
The back coils and compressor can get quite hot. Be sure to add a fan in the back to keep air circulating away from the equipment. Take a tip from your local liquor store, listen when you go into their walk in coolers, you’ll hear a chorus of fans. The more, the better.
Remember that this is a heat game. You want to remove heat and that heat flow goes from the freezer plate to the back coils. You’re not making the box colder, you’re taking the heat away.
In some cases, people have added 3, 4, 5 or 6 fans to keep air circulating at a maximum. That is key!
Also double check your door seals, stop opening the door to check temperature, be sure you insulated UNDER the floor.
Use a wired thermometer to measure temp with the door closed.
You can always add another interior layer of insulation. My keg box had a lot of extra room, so I simply added another 1.5″ layer and foamed all the corner seams. The smaller space and added insulation helped get it down a few more degrees.
My latest recommendation is to go with the new low cost compact freezer units. Same size, about the same price, but they get colder than a normal compact fridge.
If you have a keg in the box, remember, it will take a few days to get down to temp. Always buy your kegs cold, if you can. My liquor store sells both warm and cold, just like cases of beer. A cold keg will reduce the load on your system greatly.
Hope that helps…