I have a recipe that I use for my fresh strawberry and raspberry fillings that I get rave reviews on all the time. The only thing with fresh fruit fillings like this that you have to be careful with is them "oozing" out. I always cut down into the cake a bit (maybe 1/2") and create sort of a damn around the edge so the filling rests inside just to be safe. A buttercream damn may not be enough, but you can try it and see what works for you.
Anyway, I make my strawberry/raspberry filling like this: (and forgive my estimated measurements. LOL. I tend to just throw it together)
Cut up some of your strawberries (or raspberries. the recipe is the same for either) to make the "juice", and set the rest aside
Throw the cut strawberries in your mixer
To the mixer, add fresh squeezed lemon juice (1 to 1 1/2 lemons)
some honey (about 2 tablespoons)
some pure maple syrup (about tablespoons)
1 teaspoon vanilla extract
Mix it all together until the fruit is basically juice. (I usually cover the mixer with a hand towel so juice doesn't go flying everywhere). It's okay if there are some small chunks but you want a good amount of juice for the base. If I had to estimate I'd say I probably end up with about 1 cup of juice. Taste it and make sure the flavor is what you're looking for. If you need to, you can add more honey, syrup...whatever.
Pour the juice mixture into a tupperware bowl and add about 1 to 1 1/2 tablespoons of cornstarch. Close the lid tightly and shake it until all the cornstarch is dissolved. I just do it this way so I don't have to worry about lumps
Pour the mixture into a small saucepan and add about 1/4 cup white sugar
Stir with a whisk and heat on medium high. I don't necessarily stir it constantly at first but you don't want to walk away from it. Stir it enough to make sure it's not sticking to the pan. I heat it until just before it starts to really bubble (and at that point I do stir it constantly) and then I take it off the heat and set it aside to cool. It should thicken and stick to your whisk or spoon but it shouldn't be super thick like jam either. (Unless you want it a jam consistency. Then by all means, thicken away) Obviously you just don't want it to leak into or out of your cake
Slice the rest of your strawberries into strips. Once your juice mixture is cooled to room temperature, stir your sliced strawberries into it.
If you are worried about it seeping into the cake, you can always put down a thin coat of buttercream before you fill with the fruit mixture.
This filling freezes and thaws just fine :)
Hope that helps!