Step one in building a hotbox is to build a box. Genius right? I built mine out of some scrap OSB I had in the Strategic Wood Reserve (a.k.a. the rafters of my garage) The size of the box doesn’t matter too much as long as your skis fit inside. I built my box tall enough that I could use the top for a wax bench. I’ve seen others that are much smaller that also work quite well. It's difficult to see in the above picture but I hinged one side of my box for loading and unloading skis. I've seen others that just have a removable top. To load or unload skis you simply lift the top off. You'll also want to build a couple of supports for the skis to rest on.
To further combat hot spots I’ve placed an old box fan inside my box. It’s positioned in the middle, faces up and is raised off the floor of the box so air can circulate below. My thought is that this huge fan will turn the inside of my hotbox into a warm tornado. I figure with all the air moving the chances of any one spot getting too hot are minimal. Now that I think of it it’s the same concept as a convection oven.
At this point you have a box, you have a place to rest the skis, and you have a way to heat the box. Before you throw your skis in and fire it up it’s important to calibrate your hotbox. After all, you want to make sure your skis get warm enough to do some good but stay cool enough to avoid damage. 50-60º C seems to be the magic temperature for hotboxing. It's warm enough to allow wax absorption but cool enough to keep from damaging the base. (see part one of this post for a more in depth discussion of temperatures and base damage) 60º C is also cool enough to keep the rest of the ski (core, adhesives, top sheet, bindings, etc.) safe.
To help calibrate my hotbox I purchased three inexpensive indoor/outdoor thermometers. They are the type that have a long wire lead with a sensor on the end. The display unit is meant to sit inside with the wire and sensor running outside. They claim to be accurate to +/- 5 degrees. I figure that's close enough for me. I’ve suspended the sensors from the top of my box. One at the tip, one at the waist, and the third at the tail. They hang down so that they are even with the bases of my skis. If you look closely you can see all three sensors in the picture to the left. I mounted the display units on the outside where they would be easy to see. As you can see they look pretty impressive like some sort of spaceship control panel.
I’m a little OCD about this thing so I have a few rules I follow while running it. First, I never run it unless I’m home. I don’t run to the grocery store or take the dog for a walk while it’s on. I believe it’s pretty safe but I don’t want to take a risk. I also keep a fire extinguisher on hand. Again, the chances of the thing catching fire are very low but why risk it? I also clean it out with a ShopVac once in a while. I figure dust and sawdust are flammable and I should remove them from the mix. The other thing I do is check the temperature pretty often. I’ve never found it outside my acceptable range but again, why risk it? It’s easy enough to walk downstairs and peak at the thermometers once in a while.
Most of the material I've read suggests hotbox treatments should last at least 45 minutes and probably max out around 6 hours. There are two important concepts to keep in mind while deciding how to use your hotbox. (These are taken right from my conversation with Nat Brown so credit where it's due!) First, wax need not be molten to bond to the ski base. It can absorb at temperatures well below it's melt point. A hotbox isn't going to keep wax in a liquid state on your ski. The second concept is that wax absorption is a product of time and temperature. This means the higher the temperature, the faster the bond between wax and ski. Lower temperatures will require more time to archive the same bond.
Good Luck and Happy Skiing!