Honestly, unless you're down with – and have the looks for – outright prostitution, then the closest thing you're likely to manage is the equivalent-paying full time line job, where your focus is mostly on deliverables that you can in whole or in part outsource.
That way, you can at least "free up time", which you can spend secretly working on your startup from the dayjob office.
The problem with your skillsets would be ramping up a steady stream of freelance business that replicates a part time job at exactly the level of hours you wish to work. Realistically, jobs will come in bursts, at least at first, so you'll often find yourself fighting the urge to get them done and out of the way, versus honoring "regular startup time investment".
I wish you luck, and if you do find a really creative solution, let me know! I'm down to searching the couches for change…