I do not have a DIY cage, but I noticed that Dragon Strand has several ledge options for DIY cages with 16", 18", or 24" sides. That being said, I imagine that these are measured and cut specifically for DIY cages to ensure a correct fit.
*When I purchased my set, I asked for something that was a little customized to my cage which is wider than it is deep. Bill made sure that what I was asking for would fit correctly before selling it.
The set I have fits very well. There is no overhang on any of the cage sides. On the interior of the cage, the ledge itself is about 12" long. This is the case no matter what the width of the cage side is. On the exterior of the cage, the back support is the length of the side of the cage you order it for. There is also a spacer piece on the exterior. This is the same length as the ledge itself, 12". The spacer ensures there will be no pressure on the screen itself.
I am very happy with these ledges and they are very, very sturdy! Hopefully someone with a DIY cage has tried them and can give you their thoughts too.