I'm not sure if the
ZODI Outback Gear Extreme SC Hot Shower was mentioned or not. It holds 3 gals., will heat water to 100°, and the picture pretty well explains it's operation, It's rated 4.5 stars on Amazon, but at $161.90, it's a little pricey.
We used to build a form of this way back in 80's, using a 2.5 gal. Pressurized Water Can Fire Extinguisher. A lot of the newer ones have a plastic base to make it stand upright, but try to find one that has a metal base so it can be heated on a stove or fish cooker, etc. These have a pickup tube inside, so the can must be used up right for it to work. You can find these used on eBay, thrift shops, yard sales, surplus stores, etc.
These have a tire type valve stem mounted on the valve assembly, and can be pressurized to 100 psi. with a compressor or tire pump. Being a retired Firefighter, I know for a fact these things will shoot a 1/4" straight stream 25' for a good 1.5 to 2 minutes, so with the pressure reduced down by a diffused sprayer and intermittent use, it will last for a good bit.
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The hose will unscrew from the valve and you can replace it with a barbed fitting with 3/8" NP threads (IIRC). Then you can attach whatever hose and sprayer you want. Just remember, if you pressurize the system to 100 psi, and since water does not compress, it is coming out of the hose at 100 psi also. Pressurizing to 100 psi will allow you to use all of the water without having to re-pressurize, so what you may want to do is put a regulating valve inline where the hose connects to the valve assembly. This would allow you to adjust the system until you get it where you want it. Also, you need to make sure the hose and sprayer you use is capable of handling the pressure with the sprayer closed (static pressure), that you end up with.
The valve assembly is only screwed on hand tight, so it will come off fairly easy to fill it once the pressure is released. There is usually a fill line on there somewhere because you need a certain amount of space left for the air to compress in. You can heat water externally and then pour it into the can, or fill the can and heat it on a burner of some sort. Even though the can is very reflective, it will heat up some on a hot sunny day. You could paint it, but if you plan on still being able to heat it on a burner, be sure to use a heat resistant paint and don't paint the bottom.
I had thought about installing a thermometer similar to the ones used on BBQ grills, but there is no way to really get inside to secure the nut and washer, plus you risk creating a leak point. Generally, if you heat it to the point you just can lay your hand on the top part of the can without it burning you, it's about right.
