optimal placement of fixed length items on a given length
Your problem is NP-hard.  The decision problem version of your problem is to ask whether there exists a selection of equipment achieving an excess of $e$ or less (given $e$ as input, and all the lengths).  The special case of $e=0$ is exactly the subset sum problem, which is NP-complete.  Therefore, the decision version of your problem is NP-complete, so finding an optimal set of equipment is NP-hard.  In other words, you cannot hope for a fully polynomial-time algorithm to solve this problem.

If the lengths are not too large, you can adapt the standard dynamic programming algorithm for subset sum to solve your problem as well.  It applies almost directly: the dynamic programming algorithm tells you, for each integer $\ell$, whether there is a way to choose equipment whose lengths sum to exactly $\ell$.  Now you can check whether there is a solution for $\ell=100$, $\ell=99$, $\ell=101$, $\ell=98$, $\ell=102$, etc. (in that order) and take the first solution you find.  The running time will be tolerable if the desired length (e.g., 100) is not too large.