For all these drawers, I'm going with a 6mm (1/4") plywood base. While there are a few variations on this, a dado stack, router bit, or regular saw blade and moving the fence over are the best way. These are plywood drawers - they'll have a false front put on them, so there is no need to do a stopped dado to make it look prettier.

I've cut a slightly (~0.5mm) oversized slot. For the context of these drawers, it doesn't matter if they're not perfectly pretty, but it makes bulk assembly much easier with inexpensive plywood as the bases have a tendency to either bow a little between cutting and assembly, or just the fuzzy tearout can stop it seating properly.