Button = {}
function Button.create()
    local new = setmetatable({}, Button)
    new.x = 0
    new.y = 0
    new.w = 200
    new.h = 80
    return new
end
function Button:draw()
    paintutils.fill(self.x, self.y, self.w, self.h)
end

btn = Button.create()
btn.x = 30
btn.y = 20
-- Or have those as params to create()

-- Here's how you draw a shotload of buttons:
for i = 1, #mydrawables do
    mydrawables[i]:draw()
end