What would happen if you turn the propane bottle on it's side? Is the diameter at least an inch less than the height?
Of course, it would fit but...
If you turn a propane bottle on it's side, you need to make sure the relief valve and outlet is sitting in the vapour portion of the propane, not the liquid portion. You don't want liquid propane venting out when in storage, or feeding into the hose when in use.
Consumer tanks are not designed to lay on their side, because the valve/relief valve are combined and are in the middle of the tank. Some bigger tanks, like those used on forklifts, can sit on their side due to their design.
Also, I do believe that there are other small refillable propane cylinders out there that are different shapes. Some of them are made from materials that are more rust resistant than steel.
I searched locally and online.
There is a 4.25 lb tank but it's almost the same height (just under 12 inches, but still too tall) The common 10lb tanks are the same height (12 inches) as the common 5 lb tanks.
I saw a 5 lb horizontal aluminium tank years ago, but can't seem to locate one.
You can get aluminium tanks for a premium, but I've left steel 20 lb BBQ tanks outside all year, -30C to +35 C, rain,sun,snow, and they always date expire before the tank rusts badly.
That 5 lb tank cost me $35 new.
In the old days I could easily/cheaply get the tank recertified but with the advent of simply exchanging empty tanks for full ones, it is even getting harder to get one of the 5 lb tanks filled, nevermind tested for a reasonable price.
Worst case I use a spare can of spray paint and touch it up. Heck, by the time the tank rusts badly enough, I'll likely have a different boat.