Project Costing - Setting up Budget Items - Need to setup only one Budget Item Irrespective of PC/GL

We need to setup only 1 budget item in Project Costing as budgeting team does not want to budget by Source Type or Category or Sub Category. They want to budget at activity level and not at any other granular level. Also, there are no GL chartfields that would be common for all cost transactions. This means, I cannot enter a value in any of the PC/GL fields other than the wildcard %, while setting up the budget item, and this will not let me save the budget item.

The option that I am thinking of suggesting is to set up 1 budget item per GL Business Unit, and select the corresponding budget item while entering budget details on a project.