[BrianG]

This is a neat mod! Unfortunately, almost all my Spekky receivers are the older opaque gray box style where there are two PCBs inside, so no extra room for a cap.

For future reference, those empty solder holes on the edge may be an alternate place to solder the cap. I can see there is at least a ground point, and I imagine there is a power point there too (would have to do some circuit tracing to be sure).