What type of bracket to use between furnace and evaporator coil unit?
it is normal to have a flex fitting between the furnace and ductwork.
I say normal because that’s how I was taught and what I usually see.
This really reduces ductwork noise.
I use a vinyl coated fabric reenforced material clamped to the duct create a flex bulge and clamp to the furnace.
I would use caution creating a solid connection to the furnace if you have metal trunks think of a base drum that’s what the ductwork may sound like especially on startup and stop as the fan motor spins up and down there is usually a speed where it is out of balance and the noise can be irritating.
You can use metal straps to connect them but I would recommend a flexable one. (Not tape) use caution where you use screws make sure you won’t go through and hit wires, refrigerant lines, exhaust. You get the idea use caution if you strap.