How do I construct an investment contract for a sole proprietorship myself?

You hire a lawyer.

That’s the end of the conversation.

I’ve seen founders do too many dumb things to save money. When it comes to legal matters, it is NOT worth saving pennies to lose pounds.

I know you hate to pay lawyers, but the downside is disproportionate.

I’m not a lawyer so I don’t care… I’m just being honest.

One thing you could do is to hire a fundraising consultant who sets out the commercial aspects with you (typically cheaper than a lawyer) and then tell the lawyer to make exactly what you need.

Having said that… expensive lawyers (who are specialised) are worth their hourly fee.

Frankly, if you can agree with investors that you do ‘boilerplate terms’ then you just need a boilerplate. Legal costs mount when you keep negotiating and marking up the legal agreements

Lawyers at an early stage can offer you deals. They only make money when you are large and use them to do a lot of things.