what would be the best and cheapest way to convert a flat bottom wing to a laminar flow, the wing is made of foam.

Turn it around backwards?

There really is no easy way to change the profile of an existing foam wing. There actually aren't too many hard ways either. And none that are worth the effort.

The shape of the airfoil has little to do with whether the flow is laminar or not. Also you need to be careful of what you wish for. It has been shown in the past that for our model sizes and speed that the flow is primarily laminar. But this is what leads us to suffereing from big separation bubbles and the drag that goes with that. The cure for this is actually to add turbulators to convert the laminar flow early on the upper surface to avoid this and extend the high lift coefficient performance. So your foam wing's performance could well be hurt by adding a slick and shiney covering depending on the shape of the airfoil. And "flat bottom" is far too generic. There's multitudes of "flat bottom" shapes thanks to the creativity of modellers and their desire to avoid using correct "brand name" shapes.

If the goal is to make your model fly faster the only way to do that with noticable results is to try some ballast. Some of the slicker versions of the family of flat bottom airfoils will respond well to this and others will just sink out of the sky. If yours sinks like a greased manhole cover then the only alternative is to go to a new wing that has a lower camber airfoil. Typically this means a new model since few bother to build anymore.

That help?