RE: sheeting foam wing ?
Everything Newc said plus..........
The vacuum system will help retain the sheeting in the right orientation to the core. Loading bricks or sacks of corn or whatever you've found to pile onto your wing is not going to keep things lined up, it's going to contribute to shifting the sheeting around from where you placed it.
It's really not easy to set up two sheets of balsa that're slick with epoxy onto an almost weightless foam shape and then place that result so it'll keep its orientation as you start to load on your weights.
You aren't going to be able to see if the skins are aligned as you slowly stack your weights. And the building load being erratically placed is going to make the skins want to shift on the core. And you won't know it's happening.
However, once you've collected whatever it is you plan to use for weights, they'll be readily available for a 2nd try as soon as you open up the surprise package when it's cured. How do I know? Ain't saying, but I promise that I will never again try that.