A flusher would work at low rpms if, well, flushing the motor -- if there is one to fit or one that can be cut to fit. It's more convenient in the driveway. BUT, the plastic trash can is better overall with a little more effort. It's all highly technical, of course, but I recommend a plastic can with wheels to help a little in slipping it under the lower unit. Also, you have to be able to drain it before pulling it out from under the unit. I have a pvc hose faucet at the bottom on my current model (have used pvc thru hulls before, with screw in caps -- couldn't find one this last time). Fill well above the engine intake, maybe up to or nearly up to the top of the container. I also keep a hose at the back of the leg well out of the way of the prop, and keep it running to insure there's plenty of water and it doesn't overheat from the exhaust.
advantage is it's less noisy (good for neighbors), and also gives back pressure if trying to test or tweak. Can test out shifting (in adjusting carb idle, for example), but shouldn't rev too much beyond high idle, and then only for short bursts. I've always done this without a test prop, largely because I'm cheap and have a lot of different motors. I think that's probably fine as long as you are not gunning it, but that's another shadetree opinion.
