Project does not have a native way to handle a cost-per-day resource. One workaround is to use a custom cost field (at the task level) to calculate the number of days and multiply by a rate. Reports that show cost by task can be modified to show this custom cost field.
In this example, the resource "worker" has an hourly rate of $50 and the "scaffold" resource has a rate of $0. The tasks are set to Fixed Work so that increasing resources reduces the duration. The scaffold resource work on tasks 1 & 2 is set to only 1h so that it does not constrain the duration of the task. If it was set to 100h like the worker resource, you would have to manually keep the resource units in sync.
Cost1 field has this formula:
IIf(InStr([Resource Names],"scaffold")>0,500,0)*(DateDiff("d",Fix([Start]),fix([Finish]))+1). The first part checks to see if a scaffold resource is assigned, and if so, sets a daily rate of 500 (note this rate could be stored in a number field). The second part of the formula calculates the number of days, taking into account that if the task starts at noon on Monday and finishes at 9:00 AM on Tuesday, that it is 2 days.
Cost2 field has this formula:
[Cost]+[Cost1] which simply aggregates the native costs for the tasks with the scaffold cost.