I assume you mean making the 45 cut on the 18.5 side...?
If so, and you have a problem guiding and supporting at the same time, you might consider a fine blade on a circular saw and an edge guide...?
If you're talking about the slot for a spline between the wide (tops and bottoms) and the vertical sides, a handheld trim router with a slot cutter blade and edge guide will do the trick for that...a table route will give you the same support problems...
If so, and you have a problem guiding and supporting at the same time, you might consider a fine blade on a circular saw and an edge guide...?
If you're talking about the slot for a spline between the wide (tops and bottoms) and the vertical sides, a handheld trim router with a slot cutter blade and edge guide will do the trick for that...a table route will give you the same support problems...