This is an update from yesterday's post. I've got a much more viable (and more or less final) configuration now.
Step 1: Buy a Coleman Party Island ($50 + $20 shipping) = $70.
Great. They claim it can hold 8 people / 2000 pounds but that would be a very cozy and barely-staying-afloat crowd. Since the island only has one main air bladder, it tends to move around a lot, which doesn't make it very stable. You can't really stand on it as is.
Oh, and there's a big hole in the middle. Let's provide some support there for the layer we're putting on top. Put something inflatable and roughly 20-25 inches wide there. A beach ball, a partially deflated exercise ball, a bin, or any other flotation device will do. I used a $5 bin from Home Depot.
Check. next we're going to put something down so that the upcoming plywood layer doesn't pierce any splinters through the skin of the island. A tarp would likely be okay (and hey, it's waterproof!), but thicker is better. I used some old foam tiles. It is worth putting something thicker across the plywood seam that will be added later. Cost: $5-$30 depending on what is used. Chances are there's something that is lying around your house that will work. If not, grab a tarp.
Next, we add the plywood layer. The plywood layer is designed to distribute the weight of objects on the platform so that you can stand up, pitch a tent, or do other things the platform wouldn't otherwise allow. Since the platform is octagonal, I made two pieces of plywood that together make an octagon. Each piece is 3'x6' (the largest that fits through my trunk with the rear seats folded down) but could be a few inches bigger.
I had to buy two pieces of 4''x8' plywood to do this. I used 1/2" plywood, which is on the thin side but seemed to work okay. Total cost for wood: $24.
The potential for poking through the inflatable is greatest at the corners of the plywood sheet. So... just round all the corners a bit with a saw. This will distribute the force more evenly. In order to further prevent popping. buy some pipe insulation ($8 worth will cover both plywood pieces. This will ensure that the edges won't place undue force on the inflatable.
Now add the two plywood pieces to the main platform. I was lazy and put some thin flat sheets of fiberboard under the seam between the pieces of plywood as a minimal way of keeping the two pieces of plywood together.. However, the two pieces can be connected properly with a thin strip of plywood along the top and some bolts (get a carriage bolt or something like that so that the bottom end of the bolt is smooth and doesn't puncture the island). I'm going to properly connect the two pieces of plywood next time I'm in Techshop.
I did not put foam tiles on the left side of the island, so the second plywood piece's edge (with its foam protection) was directly touching the island. It was nice validation for the force distribution theory and it's why I'm comfortable recommending a thin tarp as the lower layer.
We tested having two people on the same side of the island (same piece of plywood) and it did just fine aside from some minor cracking noises coming from the plywood as we applied heavy pressure to a couple of points. Total people-weight was close to 400 pounds. I'm comfortable recommending it as a lounge for up to 4 people.
However, with the hard plywood top it totally can provide space for 2 and maybe 3 people to store a weekend worth of supplies and sleep. Now it's not a problem that all the houseboats are rented. :-)
OK, let's review total cost:
Hole filler: $5
Foam/tarp layer: $5-$30
Pipe insulation: $8
Proper connection of the two halves: $5 worth of bolts, nuts, and washers, and the scrap plywood from the initial pieces you bought.
Total cost: $117-$142.
Cost per person: $58.50-$71
And... all the pieces will fit in a subcompact car along with your food and camping supplies for a weekend. Yippeee!
Update post-Ephemerisle. It turns out the Coleman Islands can work, but they're easily punctured. You have to take really good care of them or bad things will happen. One puncture hole could ruin your whole day.