Elevated beds

Hello!  First post - I have a 10 x 2 ft  wooden base that is 26 in. off the ground and I want to place a 6 x 2 box on top.  (Back issues) My thoughts are plastic on top off the table top then a cedar box 12 in. high.  Not sure how it will drain?  Should I set the box on top of some material to help it drain?  Any suggestions are appreciated - thanks a bunch!

I've two back , a left shoulder and left knee issues .. Bee dig Square foot gardening in both modes for a long time till I move her when I moved onto MM ..soil less growth medium aka " Mel's Mix ".

  My home forty is on a slope ,  measures about 80 yards long by 18 yards wide with an 8x 18 yard bungalow  set back in the middle 8 yards from the road ,  it drops  4 feet from front down to back and 3.5 foot across it dropping down to a low of five feet in the rear LH corner as our place is built on a hillside . 

One side of my brick built raised beds  is 20 inches high where I raised the land to level things off for a flat green lawn of grass , the other side wall of the beds on the sloped land is 36 inches high , I'm 5'10  . All the tops of the beds are at the same height level .  I had 1 yard wide concrete paths  put in all round the outer edges of the beds the lower side wall are path less save for cut lawn grass. 

In truth It would have been much better if the beds had a top rim height of 40 to 42 inches , for at that height I can lean on the walling and my hand on the gammy arm  would be able to rest on it to support me as I lean forward rather than actually leaning over like I have to do on the 20 inch walling . I still have to lean over slightly on the 3 toot high walling.  

The shorter walling  is so painful after ten minutes or so I have to stop working on that side . The higher 6 inch high side  gives me a longer play time but still becomes painful after 30 min of weeding or planting things out  in the beds . 

Have a look at using a builders plaster mixing trays or two they don't rot like wood are about 6 inches deep and are 3x 5 foot  ( ish ) does and can be rimmed up in timber if you feel inclined for having MM directly next to wood  tends to have a wicking effect  with the wood drawing out a lot of moisture . Plants grow best if the nutrition is good and the moisture level constant .

Think IIRC a site member called Boffer  used them very successfully in raised bed form several years ago ( ten or so ). 
Your USA version of this sort of ebay UK thing   item code 254719685500     your US prices will most likely be a lot cheaper . 




 I feel you'd only need a few 1/4 along the sides the top of them being about 3/4 of an inch up the side wall from the base to drain off any excess water .. the very bottom  being slightly wetter than the material above it is useful as it helps make a water sump for when it's a lot dryer . I'm saying this because it is how I have made drainage holes in my 8 gallon  tomato growing tubs that grow big & cherry tomatoes, egg plants , peppers hot & cold , cucumbers etc  very well in our glasshouse .