ACA1234 cooling in a tight case

If you're thinking about cooling in a self-designed case, you first need to make up your mind about the direction of air flow: Where does fresh air enter, where does warm air exit, and how do you make sure that the air will actually take this direction?

My guess is that it currently enters the back and exits the top, but that won't pass by the CPU. In other words: You are accumulating warm air around the CPU, which may lead to problems.

If you want to skip on a fan, I'd add ventilation holes on the bottom, add a few rubber feet so you actually have a way for air to enter the case. Further, you need to close the back, so you truly have an air flow by the simple fact that warm air goes up, wants to exit the case at the top vents. If the vents at the bottom are properly placed, fresh air will pass by the CPU.

Whether or not you need an additional heat sink on the CPU is subject to the amount of air flow, which of course will be reduced by the flat cable.

Since you're designing the case on your own, you're free to add little walls to guide air. These will of course be a lot more effective if you go with a small fan. However, there's the problem of air entering from the A500 side, which may cause a headache...

Case design is an art of it's own. Not exactly my strong point!

I have to be honest here: I can't say, because I do not see how you're making sure that fresh air really passes by the critical components. Remember that it's not only the CPU, but also the ACA500plus and the X-Surf-500 that you need to keep at temperatures below 70 degrees Celsius. I don't want to give a false impression of knowing the amount of air flow in your case, the air guide strategy, heat sink performance and Theta of the glue you intend to use.

There's a whole chapter about "thermal considerations" in the 68030 data book that you can consult. In general, the CPU should be least problematic (compared to RAM and logic), as the key temperature to look out for is the junction temperature. That's the area directly under the silicon die, and if I remember right, it needs to be kept below 165 degrees C. There's a whole string of heat transfer points until you reach the "outside world", all specified in the 68030 data book. In the end, it's a number of calculations that you need to make - after you've found out how much air per unit-of-time is passing by the critical components. LibreOffice Calc is your friend

I have some tinkering going on myself inside of a GVP A530 case, which is similar in concept, but designed 30+ years ago. It was made to handle a 1" 3.5" spinning drive and a 68030 running at 40-50MHz, plus TTL board logic with PALs. Maybe what I remember from then can help.

The design pulled in air from the front via the base vents (1.5cm bend up of metal material, vertical) and exhausted it at the rear of the unit with a 12v fan. Ribbon cables were left at the rear where they might helpfully obstruct airflow leaking out the rear DB25 or DIN power plug ports (also in the base, 2cm tall at that side). The case had an external PSU for 12V power for the disk and fan. The system also had an option for a vertical slot card with logic (more heat from a 80286-16 and 512K DRAM) on the one side. This was the same ergonomic case designed for the Series II HD8. I remember the engineer checking things before it was green-lighted.

Consider cable the routes and overall cable management. This was always the challenge in the 1990's and 2000's PC case / motherboard customizations.