Pre-Filling a 30" Raised Bed

I'm just getting into SFG and have 2 4' x'8 '30" raised garden beds on order. My plan is to use Mel's mix for the top 12" so I'm wondering what is the best way to fill up the lower 18". As a point of interest these beds have no bottom.
A couple of thoughts I had:
1. Fill the bottom 18" with cardboard, branches, and chunks of wood (we have 5 acres of pine trees with loads of blow downs). Then fill the spaces with sand or gravel.
2. Cut a couple of 4' x 8' sheets of plywood to fit. The beds have rounded corners. Support the plywood with concrete blocks to a level of 18". If this is a good way to go, what should the thickness of the plywood be?
3. Same as #2 but set the plywood lower, then top as in #1, then MM on top.

Any thoughts or suggestions will help.

Thanks!

You only need 6" of Mel's Mix on the top of the beds.  I filled the bottom of my high beds with top soil, then topped off with MM. Another good fill is sand or a mixture of sand & top soil.  You would regret using things like cardboard and/or brush for years to come as it decomposes and the soil level in the beds drops by inches at a time.

PK,  What OhioGardener wrote.

Fill the lower part with inert top soil or washed sand. You only need 6-7" of the Mel's Mix.  Doubling it up is twice as expensive, both to originally make and then to amend every year.  It does not work any better than 6-7" of properly made and annually amended with composts MM.

Regarding options #2 and 3, your beds are already 30" tall.  That is a heck of a lot of weight of moist fill for any type of bottom for the bed.  If the day comes that you want to make table top beds raised on cinderblocks, 3/4" plywood lasts about 5-8 years.  Drill 1/2" holes per square foot.  Support every 2' in both directions.  Use 2"x8" lumber for the bed sides which is enough for 6-7" of Mel's Mix plus some straw mulch.

Some pics of TT beds.

Mine


Mine, overkill with 1" holes.


Pennsylvania SFG gardener.


Oregon SFG gardener.





Each child got to choose the paint color for these raised beds made with cement pans.

I've noticed that it is trendy to have super deep beds in the garden. I guess this can help those who can't bend over to ground level and it can look attractive. If I remember correctly, the general best item to bulk up under your Mel's Mix is sand, according to the SFG Method. Whatever is used, I would recommend putting some landscape fabric between your substrate and your Mel's Mix so they don't commingle over time.

Definitely recommend using the weed fabric to keep the sand, etc. and your Mel's mix separate.