I do this for my vacation / slush fund, I have a Schwab account where I transfer any excess from my checking account each month.
I max out my 401k and HSA and IRA.
Then budget a 66% savings rate on my take home pay.
Add in “mandatory” spending of electricity, food, internet, insurance.
At the end I am left with $100 a week of misc spending.
Mentally, the “slush” fund has really optimized my spending, knowing whatever I can save in food, electricity, misc will go towards a more non-mustachian purchase such as great vacations or home improvement or a nice dinner...guilt free because I am already comfortable with my savings rates. It keeps a high level of control and also turns it into a game.