Leaf springs are a very popular material for modern do-it-yourself crossbows. The availability of leaf-springs is very good and the spring steel they're made of is a good material for heavy-weight bows. Also, making a leaf-spring bow does not require much skill, as it only needs width tillering.
All this said, the characteristic hole in the middle of the leaf (and the bow) is somewhat problematic:
The best solution would be to leave the center of the bow artificially wide, thus compensating for the lost width (due to the hole). This can be difficult, as the leaves tend to be rather narrow (60mm or less), and width is consumed by the nocks and tip rise. So, oftentimes the only thing that can be done is to stiffen the center of the with other means. If this is not done, the bow will break in the middle much earlier than if it had been properly supported. This, coupled with the improper tiller is probably the most common cause of leaf-spring bow breakage.
Stiffening the middle of the bow Edit
Bow irons coupled with a very thick (5-8mm) steel plate laid on bow's back will help somewhat. Second thing one can do is use a thick bolt and a nut to make the immediate area surrounding the hole more rigid. Best results can be obtained by combining these two techniques, shown below. First from above:
Then from the above and behind:
And the from above and front:
The front-end of the stock has to be adapted to receive the end of the bolt and the nut, like here (stock upside-down):
This is advantageous (if done properly), as it allows anchoring the bow to the stock easily. Also, if the stiffening bolt is slightly narrower than the hole in the bow, the bow's position relative to the stock is easily adjustable - within narrow limits of course.
Instead of a bolt and a nut, you can use a simple rivet.