John, had this problem for years.
In my younger days, a lightweight REI foam pad was all I needed, then came the Thermarest.
I found that my maturing body needed MORE padding, I was looking at a THICKER Thermarest, but the weight is ridiculous.
Fortunately, I never threw away the old foam pads, I use them for sleeping in the back of my truck.
Hey why not just add the foam to the thermarest, since I have an external frame pack, there was no issue with bulk.
I have been a happy camper ever since, and saved myself some bucks too!
Oh yeah, one other benefit, I no longer slide off the thermarest, I put the foam on top of it, and both the bag and the thermarest stick!
Also see Jon's first post in the new GEAR section, he talks of his new sleeping pad there.