New Tent!

HBH:
I have used a piece of Tyvek for a ground cloth on several trips and found it to work great. If you use one, cut it to fit the size that you want and then I would recommend washing it in the washing machine. This will soften the material, but better yet, it will quiet the material a bunch. Turning over while sleeping on Tyvek will create a great deal of noise and will usually not only wake you up, but probably those around you to.
If you want to try something a little lighter, you may want to look at using a piece of plastic painter's drop cloth. I have used one as thin as .7 mil thick and it worked fine for an overnight trip. It is not very durable and may not hold up to 8 nights' of use. A 3x8 piece of this material only weighs 1.5 oz.
There's a couple of choices for you,
UL

In my experience, the painter's plastic does work good. Just take a few strips to patch up the inevitable holes. The tape won't add much weight and you can still get out lighter than a piece of tyvek. I would go with the tarp and painter's plastic, but that's just my two cents.

Somwtimes I take a small piece of Tyvek to use as a sit spot, but usually I leave it behind. It is something that I can leave behind, but others use them everytime.

Dwight

I have a bag that I've made that has a removable back padding panel that I use as a sit pad.

I use a small piece of an old closed cell foam pad for a sit pad. May be not as light as Tyvek, but feels better to my bony old butt. LOL!
UL