I do not take to much stock in perfect well thought out planters, this is seat of your pants get it done
everything even the screws have been recycled,i live in the country and nothing is perfect after the elements have battered it into submission.Actually it looks even better, nature at its best. I have made two planters one taller and one shorter even two colors (just the paint left from another job or two).

Step 1: The Oil Drum

A Word Of Warning the contents of oil drums even when empty have the potential to explode
i add a gallon of soapy water to the drum and reseal it.rolling it around the garden for ten minutes dislodges most of its contents.Empty it out into a tray to dispose of later.leaving the lid off afterwards for 24 hours, a visual check with a torch to make sure its empty and if any smell lingers. I only use petrol or paraffin drums as i feel confident with them.If it still smells try washing again until there is no residue left.

Step 2: Cutting the Drum

cutting the drum is easy but you should use eye protection and heavy gloves spitting

steel and very sharp edges is a recipe for disaster...be safe.use chalk to mark out the

drum and a disc cutter to cut through then round off the cuts with the grinder.

take a drill and drill holes in each half to allow for drainage.

Step 3: The Wooden Base

The wood i have available was cast aside by a friend who had it left over from a fencing project so instead of being burnt it ended up in my stash of possibles.laying two planks side by side on the floor just place the drum on the boards so it makes the shape of a half circle , use chalk to mark the planks with the outline,put a plank to either side on its side and add 2 more lines,repeat one more time making sure to keep the widest part of the drum the same distance from the lower plank,(in short mirror what you did)use a jig saw to cut them out,you are making a cradle to hold a half of a drum for the length of the wood pick a size you like as long as it is over 3/4 the length of the drum (yes again no measurements,just keep the sides the same and the lengths the same)2 lengths or 4 if you prefer a larger base if your doing both half's of the drum just multiply everything by 2.I screwed some scrap wood to each corner of the end sections to hold the two planks together,from the edge of the wood the thickness of the planks used.Lastly just screw your boards together try to keep them flush with the top.