Central Ohio Checking In

RLR,  Welcome to the Forum from California!    Have you been able to read the book All New Square Foot Gardening?  It will answer a lot of questions, and raise others.  Using this method solves the soil problem so you can focus on the plant growing.  Also, use the Search box for a gillion topics.  We are to help.  

Oh, my, are there examples!!   If you are going to go with the SFG type of dirtless-mix, called Mel's Mix after it's creator, you only need 6" of the mix.  You can make it deeper if you want but only 6" is needed except for long-root veggies like carrots and turnips.  PS  Camprn and I are 2 other folks that raised our beds because of root invasion, and our beds were filled!!  A major job but it had to be done.  Use the Search box for Table Top and Top Hat.

My 2 cents:
1.  Remove the top 2 rows of siding.  Cut off the corner braces level with the remaining sides.
2.  Lay 2" x 4's (treated would be best) across the bed every 2', beginning and ending flush with the edges of the siding.
3.  Lay a sheet of plywood (1/2" min, 3/4" better).  Draw or snap lines in both direction to create 1' squares (as in Square Foot gardening.
4.  Set the top 2 rows you removed back on top of the plywood, reusing the corner braces that you previously cut off flush.  They should fit perfectly!
5.  Drill 1/2" holes in the center of each square you drew and one in each "corner" of the bed.  Remember that you have 2" x 4" under the corners so the holes in the corners will not be perfectly in the corner.
6.  Line the plywood with weed fabric to keep the mix from washing through the holes!
7.  Fill with Mel's Mix.

RL,  I think your assessment of my suggestions is correct.  The lower part is too weak to support the weight.  I have TTs supported by 4" x 4" treated wood.  The largest is 2' x 8'.  This is my Valentine's Day new 2' x 7'.  

The bed sides are 2' x 4" studs  Because they are thick studs, they can be bolted to the legs for strength.  

I also use plumber's tape to strap in 1" PVC frame holders before I fill the bed.  

Because there is no such thing as a level spot in my back yard, every leg seems to need a treated shim.  For you, figure out how long the lower legs need to be to create a near level condition to set the plywood  bottom and sides.  In stead of a post hole digger, just level off the individual foot prints of each post.

If you don't want to use wood legs, use cinder blocks as legs.  To fine tune the downhill block height, shim with a board of 1' (3/4"), 2" (1 1/2")  3" (2 1/2") , etc.

Here you can see how the treated 4" x 4" legs are notched on which to set the plywood and side. If you want to double up with 2" x 4"s, just bolt the shorted support piece to the full height piece.

The thinner wood you used for the sides?  Build a compost bin, or small beds that don't need strength set on bricks or pavers .  Nothing goes to waste with us, tight wads that we are!

I hope this gives you some ideas.