Take the 40L stuff with a pinch of salt - I keep trying to tell people that this is a flawed finding, but 'the true believers' don't want to hear this.
Consider: the size of a swarm influences the size of the box it is attracted to, and ultimately chooses - most people (even Finman) recognise this. But - there is no such thing as a BSS (the British Standard Swarm) !! They vary in size, with their size being a function of the colony size they eminate from (and therefore the size of box that colony was housed in). Thus - any idea of a Standard-sized bait hive must be flawed. And this is just considering Prime Swarms, for as we all know Cast Swarms are far smaller in size than Primes.
So - be more concerned with the number of swarm traps you put out, and their 'spread' around the area, then just their size.
Some people have used upturned tree pots and buckets - attach a piece of old plywood to the underside with a hole 20-50mm or so near the centre. Suspend in a tree or other likely spot.
It would make sense to attach another piece of plywood to the underside of the top ("There's a hole in my bucket, Dear Liza, Dear Liza ...") to which you could attach short bars - to become starter strips - and if you attach these to the plywood with screws from above, then if/when combs are drawn from them, they can be removed intact and tie-wrapped underneath Frame top-bars, or just 'Top-Bars', if you're so inclined.
When you consider that bees have been known to set up home in just about anything - from utility boxes to old car tyres - just about any container you can lay your hands on is worth putting out. Just try to make sure that the inside is as dark as possible, and that it smells 'right'. Old comb is favourite, scraps of propolis etc.
Best of luck
LJ