On variants of the Furstenberg set problem

Given $s \in (0,1]$ and $t \in [0,2]$, suppose a set $X$ in the plane has the following property:~there is a collection of lines of packing dimension $t$ such that every line from the collection intersects $X$ in a set of packing dimension at least $s$. We show that such sets must have packing dimension at least $\max\{s,t/2\}$ and that this bound is sharp. In particular this solves a variant of the Furstenberg set problem for packing dimension. We also solve the upper and lower box dimension variants of the problem. In both of these cases the sharp threshold is $\max\{s,t-1\}$.