The ways I've done this in the past are
1) Find some customers that are willing to hire you (or your product) but know that you'll only be free nights & weekends to support/work with them.
2) Find a "partner" (co-founder or other) that's got a flexible schedule that can help build the business while you're at work.
3) Block out nights, mornings and weekends to build the business till you have enough orders to cover 50% of your salary. This might mean 7pm-11pm most nights, and 4 hours each day Sat & Sun. Make progress (sales $$$) and momentum.
All that being said, it's risk reward. Sounds like you want to avoid taken the risk, and I get that .. but the upside is always smaller. Unless you put yourself in a position to have to succeed (ex: quitting your job) then you may never make the scary decisions that are required to build a company (like cold calling, going in debt, making a presentation, etc).
I'm on company #5 with many other side projects started nights & weekends .. so I get it - but don't be afraid to bet on yourself and go all in.