Milk cake? Tres Leches? I agree with clamshell (like this https://www.amazon.com/dp/B07CW642ST/ref=cm_sw_r_apan_i_HT2NKDGJ13VPPA0SNGW3) unless you mean tres leches because the goo will leak out.

We use 16oz square/rectangle packaging for cake slices at my work, so I would try searching 16oz.