We use big green bottles of liquid dishwashing detergent which are a pain to get the last bits out of, so for years we'd just prop the bottle somewhere under the sink to stand upside-down.  It would often fall, knock stuff out of the cabinet, etc. so I whipped up a little stand to hold the bottle upside-down.  Just keep the handle toward the front where it'll be supported.

It's not a replacement cap, so it can be used with other similar sized bottles and won't end up too messy.  I printed ours in PLA with really, really low infill (and the layers didn't stick very well either - I was new at this) but it's still held up well for a couple of years now.

The original had the nice sloped edges, but I also put together a simpler version that is untested, but just has 3 ribs for support (no idea if necessary, but I was trying something new with Fusion 360).