Evolving noun case marking from postpositions

I'm having trouble evolving the noun cases that I want in my conlang, because sometimes postpositions aren't in a place that they could be evolved from.
For example I have a sentence that looks like this:

Ma kir karab jud igad ma.
(I + give + fruit + at + friend + I)
I give a fruit to my friend.

I could evolve an accusative case by combining fruit + at = fruit-ACC , but what if I wanted to evolve a dative and genitive case?
And what there was also an adjective involved, so it would be:

Ma kir karab takaru jud igad ma.
(I + give + fruit + big + at + friend + I)
I give a big fruit to my friend.

Then I wouldn't be able to evolve any cases that I want.
So is there any way I could do this? Maybe evolving case marking from something else? I don't know, I'm still very new to this.