I have a piece of 5/8 plywood cut to approximate the floor of the baggage area. This goes down first to protect the baggage floor from point loads like feet and knees. Then a piece of 2x10 lays on that and into the rear fuse across to the next bulkhead. That one is about 5 foot long or so. Not sure, itís been a while. If I need to go further, a piece of 5/8 x 10 plywood to the next bulkhead.
This is for a sliding canopy RV7A. A tip up canopy aircraft may be harder to get the lumber in there.
Caution, if itís a tri-gear, you may need to support the tail before the helper crawls in there. A custom height sawhorse should do.
Also, you may want a small fan to push air through for the willing victim. It gets warm in there fast particularly if you have a hot light in there.
Proceed with caution so as not to require the assistance of the FD for extraction.