Hi Phil,
If you can use a small angle grinder and a saw its easy.
The blocks in the bottom are light weight blocks such as celcon/ thermalite, i picked up from wickes.
Place some blocks on the ground and place drum on them, mark round, mark another circle 15mm in and use that as cutting guide.
You can cut them to shape with a handsaw but I find a bow type tree saw the best and the blades are cheap though you will only need 1 for this job.
Place cut blocks in the bottom, the other blocks cut into 4 so each one is 110mm wide.
Get plant pot, mine was from b & Q £7.99, cut the bottom off with andle grinder 25mm down
keep base as lid.
Place pot centrally on blocks inside drum with original pot top end down, mark round, take note of heght of pot from base remove pot add the 100mm for block depth, make mark on drum at the height and angle grind drum through.
Using the blocks you cut, place them around theline about 10-15mm more into the centre, place them on end so they are300mm high but use the 100mm face pointing to the inside, leave a gap about 100mm wide between the last 2, place the upside down flower pot on the blocks and adjust as required. where you left the gap cut a slot in the outer drum in line with it for ventilation and fire access, cut a bit of block to go across the top of the blocks either side of the opening to stop in fill going down.
Carefully tip a bag of sharp sand around the blocks and tamp down gently, fill the rest up with vermiculite, tamping down as you go and put a thin mortar collar around the top.
Regards
Gary
Thanks for the thumbs up, cost
oil drum Free
1 bag sand £ 1.39
7 blocks £9.73
flower pot £7.99
edge trim £2.30
vermiculite £ 9.00 ( 1/2 bag -50 litres/ 100l bag £17.99 from Jewsons, cheapest i could find)
cement £0.00 left over bit from other work)
TOTAL £30.41
PS
My biggest fear in life is that when I die my wife sells all my stuff for what I told her I paid for it!!