I built my own out of 3/16" plate. It's not "hard", but it did take a long time to build when you are figuring out all the dimensions and designing as you go. I made mine a little different than most by designing the "sump" section inside the skid. That way I have a completely flat skid on the bottom, and the sump is accomodated by 1/2" thick polypropylene spacers inside the skid.
I would suggest having a good motorcycle jack handy, as I probably raised and lowered the tank 20X getting all the supports and corresponding bolt holes in the right locations.
I looked all around for dimensions, but didn't find anything on the web specific for a TJ, although I did find some for a YJ. All I did was set the tank on a sheet of cardboard and used a speed square to locate the outer perimeter. Once I had that template I decided on the height of the side walls and used more cardboard to mock up the sides. So basically I built a complete cardboard mock-up and then used that to cut out all the pieces from my 3/16" plate.
The trickiest part was the front of the skid. It has a 15 degree angle bent into it, to follow the contour of the tank and then bend backwards to clear the track bar. Once I had the dimensions, I took it to a local shop with a large press brake and had them bend it. The other tricky part of the front plate is cutting the rectangular holes for the hold-down straps. My plasma cutter worked well for this task.
Once the main skid was done, I glued in the polypropylene spacers and sprayed the whole interior of the skid with gasoline resistant under coating. I put a series of drain holes along the front and back edges as well as notching the front corners so any water can easily flow out.
I put the tank in and located the angle iron pieces that support the back and the front as well as the third "arm" on the passenger side front. I also designed this to raise the tank up ~ 1" from the stock location as well. In the last picture you can see it getting ready to be installed. I actually had to do a few more mods after that picture, like notching the corner of the front plate to allow more space for the fuel pump electrical connector, and I took off the plastic pieces that were on the straps holding the tank in place as they were hitting the underside of the tube. I probably could have just eliminated the straps altogether as there's no way for the tank to move since it's very tight to the underside of the tub, but I figured it was still a good idea to to have them.
It did take a LOT of time to build it, and although the cost of the 3/16" plate was only $70, I probably had 20+ hours in the build. However, it does have some features, like the flat bottom which are different than other skids on the market and it was a good project. Here's a couple pictures of the build: