Have you tried finding/repairing that slow leak? I have a Neo-Air, and it had a slow leak. I found it by standing on it while it is submerged -- bath tub or swimming pool is best. Watch for tiny bubbles. I suppose if a leak is on a seam it would be difficult, but the pads I've had have all leaked from easy-patchable spots. Must be aging/defective coating going bad. I realized storing them folded/rolled up tightly causes wrinkles, and that can cause a leak over time. I now store mine inflated.
As for a closed cell pad, seems like any would work, regardless of lumps, since you use it as a bottom-layer. Weight would be the most important to me.
