Right. This is something that looks like the final product, but isn't ready for manufacture.
* If you run a 9V battery into a linear regulator to get 5V, there goes 55% of your energy and battery life. Consider 4 AAA cells. Work on reducing standby power consumption. When idle, put the CPU to sleep until IR is sensed. (An ATmega can do this.) Things which burn battery power when idle fail as consumer products.
* "I could (and probably should) use an ATtiny for such an application but I only had some ATMega328 in my parts bin."
* Prototype plastic things with 3D printing. Manufacture them by injection moulding. The little injection moulder at a TechShop could bang out about one box a minute. Any commercial shop can turn out thousands in a day. Making the mould is either work or expense, but you come out way ahead after a few hundred parts.
Get a TechShop membership, take the classes in injection moulding, CNC I, CNC II, and CNC mill. You get free access to Autodesk Moldflow, an injection moulding simulator. Learn to use it. Design and mill your mold. Make some parts using TechShop's injection molding machine. Make a new mold if you're not getting what you want.
Completely straight sides make injection moulding harder. The tooling for straight sides has to come apart to release the finished part. But a shape that's smaller at the bottom can be made in a far simpler mould. This is why cheap items such as plastic glasses are tapered so much. Look at plastic parts around you, and notice that most of them are shaped to come easily out of a simple mould.
Consider making the outer shell by vacuum-forming. That's cheap and easy, and the tooling can be made of wood. TechShop has that capability, too. It works better for shapes that aren't as sharp-cornered as a cube, so consider modifying the shape to have rounded corners.
Both of these processes will produce a much smoother finish and a stronger part than 3D printing.
My shop recently did some work fixing some injection molds that a customer bought from a Chinese manufacturer for about that price. I was shocked by how cheap they were. Of course we were being paid to fix them because they weren't quite right, but still, $5k. Wow. If you want quality 3D printed parts, that amount of money burns up very quickly.
Honest advice? Hire someone to do it. There are so many things that you won't know about how to make things manufacture-able it would be worth your time and money to just pay a contractor/employee to optimize things for you.
(shameless plug, my username @ gmail.com if you would like to discuss what services I can offer you)
If you want to go that route, have a contractor review at various stages. It shouldn't take more than an hour or two, and most contractors will do it with the hope that if you make it big you will remember them and their services.
* If you run a 9V battery into a linear regulator to get 5V, there goes 55% of your energy and battery life. Consider 4 AAA cells. Work on reducing standby power consumption. When idle, put the CPU to sleep until IR is sensed. (An ATmega can do this.) Things which burn battery power when idle fail as consumer products.
* "I could (and probably should) use an ATtiny for such an application but I only had some ATMega328 in my parts bin."
* Prototype plastic things with 3D printing. Manufacture them by injection moulding. The little injection moulder at a TechShop could bang out about one box a minute. Any commercial shop can turn out thousands in a day. Making the mould is either work or expense, but you come out way ahead after a few hundred parts.