If you are not getting good transfer, I would suspect either not enough heat (the toner won't stick if the enclosure does not get hot enough), or a non-flat surface (either the iron or the enclosure).
1. Sand the enclosure flat. Use an actual sanding block, not your hand or a chunk of wood. I start with a low grit, like 80-100. Just because it looks flat doesn't mean it is flat, so I draw all over the top of the enclosure with a marker, and take a pass or two with the sanding block. If 100% of the marker is not gone, the box is not flat.
2. Sand the enclosure smooth. Once it is flat, sanding it smooth is fast and easy. I work my way up in grit, using the sanding block to make sure I keep the top flat. I usually stop at 400 grit, since I have had trouble with getting toner to stick if the box is too
3. Clean the enclosure. I use liquid dish soap, and then dry it. I then use acetone to remove anything that could possibly ruin the transfer.
4. Make the toner transfer. I use this paper
, available at Dollar Tree stores. 8 sheets for $1. A lot of people have used, and had success, with that paper. It works great. I quarter them, and end up with 32 transfer sheets capable of doing 1590BBs. I use a Samsung laser printer, and it works great. Toner Saver off, maximum toner, etc.
5. Transfer. Preheat the iron as hot as it goes. I find a block of wood that fits inside the enclosure, but is just a bit thicker than the enclosure. The idea is to sandwich the transfer surface between the wood and the iron, so as to not put pressure only on the edges. I put the transfer image down, put on a small piece of printer paper, and then the iron. I let it sit for about a minute with a bit of pressure, to make sure the transfer starts to stick. I then move the iron back and forth with pressure for one minute in each direction. At that point, the transfer is usually pretty well stuck. I remove the sheet of paper, and use the tip of the iron for a minute in each direction, just to be sure everything has been transferred.
6. Remove the paper. Some people just peel the paper right off, but I haven't had much luck doing that. After letting the enclosure cool, I soak it in room temperature water. Once the paper begins to absorb a bit of water, you can speed the process along by using your thumb to cut through the coating on the back of the paper,allowing more water in. Remove all the paper with your thumb, toothpicks, bamboo skewers... it is pretty durable.