I believe this backfires! You’ll subconsciously get more focused as the one-hour bound approaches (and/or less focused for the few minutes after a ping) and that introduces a bias in the sampling. If you conscientiously avoid doing that, you’re liable to over-compensate and end up with a bias in the opposite direction. I think the only way to feel confident in the sampling is to nix the bounds and do proper, ungameable Poisson sampling, per the spec.
PS: Awesome work on Cactus! Thank you for sharing it and discussing it here! I just added it to the list at tagtime.com as well.