Use 5 gallon buckets. Mother Earth News has a good article on how to do it. See link below.

Here is a link that might be useful: Mother Earth News

wertach, Thanks for that link! Reading it now, looks like that might be just what I'm looking for.

Slightly off topic: Has anybody had reasonable success growing watermelon in containers? I was thinking about giving 'Moon & Stars' a shot when spring rolls around, but the info I've found seems kind of inconclusive about how practical it is.

Thanks again!

high density polyethylene (HDPE) is what i use for the soil bench. and you can grow watermelon in a swc.
this is one of my (motherbox). first year in the experimental stages.
pumpkin plant two months ago.
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pumpkin three weeks ago.

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terrybull, That pumpkin is amazing!

Do you find your HDPE anywhere special?

i got mine from farmtek and i used 1/4 inch thick. you could get by with 1/8 inch for a smaller container. there are alot of other places that sell it in smaller sheets. just do a search. farmtek calls it polymax.

You could usw a tin can with both ends cut out in place of the PVC to holr the bench

"but I'm not sure what to use as the platform itself"

Do a further search for self watering containers or SWC.

You can cut out most of the lid center to fit in the container and support it on 5-6 lengths of 3" diameter pvc. This will make a single wall SWC. There are several other successfull variations using one container and the lid.