If an indoor antenna is the only option, and if your roof construction is transparent to RF, I would recommend something similar to what Warren shows in is first pic in post 4 but taken a bit further.
Based on some testing I've done with large HF Screwdriver antennas I would suggest the following, and when I say large Screwdriver I don't mean the POS Yaesu ATAS series or anything similar, I mean a real Screwdriver with 2" or larger coil. I like the Tarheel 100HP series but the Tarheel 75 or other large coil brand will probably work fine.
Line your attic floor with as much chicken wire or hardware cloth as possible and 20ft X 40ft is not too big.
Plant the large Screwdriver antenna mount, mast and coil in the middle where you have the most height available in the attic. You can use a cheap CB mirror mount screwed to the floor joists and overlap the chicken wire for grounding.
Instead of using the supplied whip make a large capacity hat out of wire and loosely staple to the rafters and connect to the top of the coil with a short wire. Make sure the capacity hat can move with the coil as it goes up and down.
A verbal picture from the floor or joists up would be chicken wire, mirror mount, mast, coil, short vertical wire then capacity hat wires heading out in several directions horizontally.
Most large Screwdriver antennas have about a 6ft whip and a capacity hat around 3ft dia with a 1ft vertical wire connecting the coil to the hat would be about right to simulate a 6ft whip to keep the antenna useable on 10m.
You can make the Screwdriver much more efficient by increasing the size of the capacity hat to say a few wires 10ft long (20ft dia) and a few as long as the width of the attic will allow. This will make the antenna resonate on the lower bands with much less coil inductance and increase the efficiency on lower bands but the antenna would no longer resonate on the highest bands like 10m.
I have a Tarheel 100HP on my truck and it works very well and on 40m its not that far down from the 94ft flat top dipole at 30ft on my house. When I run 500w mobile distant stations say the mobile is noticeably better and 500w is 7dB higher than 100w so my Tarheel mobile in my front yard is only a few dB down from a large outdoor dipole when working DX. This is partially due to the low angle radiation of the mobile compared to NVIS from my dipole.
You can get a lot more chicken wire ground plane in an attic compared to a vehicle and the attic is much higher off the ground so ground losses should probably be less than the same antenna mounted on a vehicle.
As zz said, an indoor antenna is way more susceptible to RF interference inside your home but if you have a lot of chicken wire laid out as a ground plane the RF exposure beneath what I described would be minimal, unless you have rooms adjacent to where the antenna is located.
Just my 2c and ymmv.
prcguy
