Best way to heat sink many resistors
One cleaner way to go is to use a thermal pad. Those come into sheets that you can cut to the appropriate size, you can then screw an aluminum heatsink to it. There is a long thin heatsink that would fit there.

The advantage of this method is that you don't have to do anything special for the SMT process and can be mounted afterward. It also avoids the mess of using epoxy / thermal compound, can be disassembled easily.
These pads come in different thicknesses from less than a millimeter to 5mm, so it &quot;squeezes&quot; when compressed, allowing for small height differences between components.
If you cannot afford screws, 3M manufactures a thermal pad that is sticky on both sides. I've been using that for years and yet one has come apart. It also comes in sheets that you can cut.