Filling a raised bed.

I'm planning a raised bed for my sister.
I want it to be at a comfortable height.
We have determined 3' is good for her.
We need only need 1 foot of soil to grow the crops we desire.
The bed will be inside a hoophouse.
I'm torn as to what should occupy the other two feet of space.
Here are some ideas I've had:
-Insulation: autumn leaves or cardboard 2 feet deep.
-Solid Thermal Mass: dirt and/rocks, possibly with peforated pipe, steel duct or steel barrels, for air-to-soil heat transfer.
-Liquid Thermal Mass: Sub Irrigated Planters.
-Air: A sturdy grow table.

The insulation option might draw rodents,  and will certainly settle.
Dirt is heavy.
Metal is destined to fail, eventually.
Water is liable to freeze.
Building to support a foot depth of soil could be tricky.

I'm leaning towards tamped dirt with peforated pipe running through it, plus a simple solar powered fan driving heat transfer.

I find rodents to be a reality in hoop houses.  This is the time of year for all sorts of outdoor displays.  I would fill in the space with scrounged straw bales.

I'd fill it with chonks of logs and chips in between. It'll settle, but much more slowly than cardboard.

I wish I could get discarded haybales, but that's not much of a thing around here.
Leaf bags are abundant as all get out.
I top dress my beds with 7with leaves,
Wood, chunks filled in with chips sounds great.
Small branches tend to create rodent friendly voids in hugel mounds, but chips fill those gaps.
I don't have a steady supply of chips!
I do have plenty of dirt.

I am reconsidering the water option.
I can make 2' tall grow table "legs" from water heater tanks.
I think I could even retain the outer sheet metal skin , but I'd probably need to replace the insulation
A table with shallow sides and a water proof bottom could hold containers.
Buckets,nursery pots,milk crates ,grow bags or anything else roughly a foot deep.

When we manually water it would drain back into the tanks.
If we have an inch or so of sand/charcoal/bark fines on the table top we could hang wicks into the tanks and maybe get some capillary watering.
Continuous circulation via a solar pump would be more reliable and very cheap.
With a pump, we could set up  solar thermal water heating on the north side/ along the ridge of the greenhouse.