What I'm searching for is a small card that has: SBPro support; a wavetable header or wavetable synth, true MPU-401...

There is no single card that can do all that. While wavetable headers are common, only the MPU-401AT offers true MPU-401 and SB Pro emulation tends to be crap. Since you already have two out of three with the SCM-15AT combo, put an SB Pro 2 in the middle slot, it should fit, and you will have perfection in a tiny package.

Still, a very nice system to be sure. Unfortunately you couldn't fit an LAPC-I in it, otherwise it would be perfect. Do they come in full-length (13")versions with four slots?

Yes, a SB2pro would complete the box in a classic way. The problem is, that some components of the SBC are too high, so I can not use the second slot.
I have another SBC with a Pentium Tillamook 266mmx that would fit in the second slot, so I could use the third one for a SB2pro. But the Tillamook seems to get hotter than the cyrix. There is an option in bios, that the cpu slows down automatically when it reaches a certain temperature: When I set it to 40°C (the lowest possible) the CPU slows down to around 70-75Mhz.
I tested it for half an hour, and temperatures were 46°C suspend mode, and 57°C "Load" (playing Star Control 1 in "auto mode"). That doesn't sound so much, but I fear

- that the temp. still gets higher, especially with another card (SB2pro) in the case, as there is no real air circulation, just the small louvers on the top

- that the build up of heat could harm the Roland cards.

Apart of the fact that I yet don't know how to measure temperature inside the case, what (cpu)temperature do you think is still tolerable for the SCD-15?

lots of Socket A coolers will fit on Socket 7. You could look for a 1U server heatsink and that would probably solve the heat problem. Also could consider a Winchip or Winchip 2. They run cooler than anything else for Socket 7, if I recall correctly, and are very compatible little CPUs (work in Socket 5 even, I believe). The only time I've had trouble with Winchip 2 was with a Gateway mobo that wouldn't post if "Genuine Intel" wasn't in the CPUID string.

And really, up to 70C should be safe for CPUs. Things may get warm in the case, but it shouldn't bother anything. It's not going to get THAT hot. 😀

Oh, and you could also run an idle app for DOS that will send HLT commands to the CPU. I believe Pentiums support this.

Some coolers:
http://www.newegg.com/Product/Product.aspx?It … N82E16835150029
http://www.newegg.com/Product/Product.aspx?It … N82E16835110012

Tanks for the hint, but using a different CPU cooler with the tillamook is no option because the cpu and its passive cooler are fixed on the board.
But I have yet another SBC with a normal socket 7, so the IDT winchip might be interesting for this...

Anyway I discovered that the SCD-15 fits in the box using the middle slot. Somehow it's a little bit wider there (it has to be the first card to be plugged in), and so one configuration with the original Soundblaster Pro 2.0 card added looks like that:

It's still very narrow,especially when you use the tillamook board instead:

The tillamook board has the advantage of a build in CF-card slot, but the image quality of the on-board vga (C&T 69000) is a little bit blurry and below that of the 486 board (Cirrus Logic chipset).

Unfortunately you couldn't fit an LAPC-I in it, otherwise it would be perfect. Do they come in full-length (13")versions with four slots?

Sorry for answering this question so late: yes I think there are, e.g. this one:
http://www.aaeon.com/PD_Products_Detail_E062A … F_TW_utf-8.html

Using industrial cases and SBCs might be an interesting approach for building a retro system, as changing the main components - mobo and cpu - is a matter of seconds. There are ISA only and PCISA (mixed) passive backplanes and for compability reasons most if not all newer boards should fit and work with backplanes that where originally designed for 80386 or even older boards (and vice versa I guess). The drawback of course could be the price: the case above might be arround 200-250$ (I don't know exactly, got mine at ebay).