Your drum liner would definitely work trimmed. The only annoyance with the drum liners and trash bags is that they're black and so wide, that it can sometimes be hard finding things in the pack, but for the price, they're tough and very much functional. When it wears out, you have a whole box more of em :thumbup: If you like the system, you can think about getting fitted pack liners for convenience and weight, and dedicated dry bags in fancy fabrics if you wish. I'm currently using a dry bag pack liner in 1.0oz/yd^2 cuben, and it's been treating me well.
If you keep your liner buttoned up (various methods -- the simple spiral tight-twist + fold/roll/overhand knot is effective for pretty much anything you'd encounter short of full submersion...and even then it'd buy you time), you stuff will stay dry. I've done lengthy day hikes in rain with pack liners and nary a problem. I have yet to do a full-on sustained-dumping backpacking trip with a pack liner, but intermittent rain on past trips has been fine (there have been times when it does indeed get annoying to don and doff a poncho). Condensation should be a non-issue. The only issue I think MAY be worth a thought, as I alluded to, is your having to carry extra water weight on a soaked pack. But I don't have any concrete, quantifiable, experience to validate or invalidate that yet.
The only vulnerable electronics I have are my phone and my camera. My headlamp and small keychain lights are all waterproof enough to withstand rain and a little soaking (usually in hipbelt or pockets). The phone is stashed away in a ziploc in the pack or hipbelt already, so no problem there. The camera (point and shoot) is usually riding on my belt for easy access, and it is protected when I'm wearing either a poncho or rain-jacket. If I ever feel that isn't enough, taking pics when exposed to the weather wouldn't be worth the risk at that point, and into the pack and liner the camera would go :thumbup: